diff --git a/core/authorize.php b/core/authorize.php
index 8a76fe4..3507f7f 100644
--- a/core/authorize.php
+++ b/core/authorize.php
@@ -23,7 +23,7 @@
 // Change the directory to the Drupal root.
 chdir('..');
 
-require_once __DIR__ . '/vendor/autoload.php';
+require_once __DIR__ . '/autoload.php';
 
 /**
  * Global flag to identify update.php and authorize.php runs.
diff --git a/core/autoload.php b/core/autoload.php
new file mode 100644
index 0000000..4909004
--- /dev/null
+++ b/core/autoload.php
@@ -0,0 +1,7 @@
+<?php
+
+use Drupal\Core\Autoload\DrupalAutoloaderInit;
+
+require_once __DIR__ . '/lib/Drupal/Core/Autoload/DrupalAutoloaderInit.php';
+
+return DrupalAutoloaderInit::getLoader();
\ No newline at end of file
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index e8350cf..ca063bd 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2719,7 +2719,7 @@ function arg($index = NULL, $path = NULL) {
  *   loader class when calling drupal_classloader() from settings.php. It is
  *   ignored otherwise.
  *
- * @return \Composer\Autoload\ClassLoader
+ * @return \Drupal\Core\Autoload\ClassLoader
  *   A ClassLoader class instance (or extension thereof).
  */
 function drupal_classloader($class_loader = NULL) {
@@ -2731,7 +2731,7 @@ function drupal_classloader($class_loader = NULL) {
   if (!isset($loader)) {
 
     // Retrieve the Composer ClassLoader for loading classes.
-    $loader = include __DIR__ . '/../vendor/autoload.php';
+    $loader = include __DIR__ . '/../autoload.php';
 
     // Register the class loader.
     // When configured to use APC, the ApcClassLoader is registered instead.
@@ -2761,7 +2761,9 @@ function drupal_classloader($class_loader = NULL) {
  */
 function drupal_classloader_register($name, $path) {
   $loader = drupal_classloader();
-  $loader->add('Drupal\\' . $name, DRUPAL_ROOT . '/' . $path . '/lib');
+  $loader->addPsr4('Drupal\\' . $name . '\\', DRUPAL_ROOT . '/' . $path . '/lib/Drupal/' . $name);
+  $loader->addPsr4('Drupal\\' . $name . '\\', DRUPAL_ROOT . '/' . $path . '/lib');
+  $loader->addPsr4('Drupal\\' . $name . '\\', DRUPAL_ROOT . '/' . $path . '/src');
 }
 
 /**
diff --git a/core/install.php b/core/install.php
index 7152fd8..e2900a2 100644
--- a/core/install.php
+++ b/core/install.php
@@ -8,7 +8,7 @@
 // Change the directory to the Drupal root.
 chdir('..');
 
-require_once __DIR__ . '/vendor/autoload.php';
+require_once __DIR__ . '/autoload.php';
 
 /**
  * Global flag to indicate the site is in installation mode.
diff --git a/core/lib/Drupal/Component/Plugin/Discovery/AbstractAnnotatedClassDiscovery.php b/core/lib/Drupal/Component/Plugin/Discovery/AbstractAnnotatedClassDiscovery.php
new file mode 100644
index 0000000..00ceeb7
--- /dev/null
+++ b/core/lib/Drupal/Component/Plugin/Discovery/AbstractAnnotatedClassDiscovery.php
@@ -0,0 +1,141 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\Component\Plugin\Discovery\AnnotatedClassDiscovery.
+ */
+
+namespace Drupal\Component\Plugin\Discovery;
+
+use DirectoryIterator;
+use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
+use Drupal\Component\Reflection\MockFileFinder;
+use Doctrine\Common\Annotations\AnnotationReader;
+use Doctrine\Common\Annotations\AnnotationRegistry;
+use Doctrine\Common\Reflection\StaticReflectionParser;
+
+/**
+ * Defines a discovery mechanism to find annotated plugins in PSR-0 namespaces.
+ */
+abstract class AbstractAnnotatedClassDiscovery implements DiscoveryInterface {
+
+  /**
+   * The name of the annotation that contains the plugin definition.
+   *
+   * The class corresponding to this name must implement
+   * \Drupal\Component\Annotation\AnnotationInterface.
+   *
+   * @var string
+   */
+  protected $pluginDefinitionAnnotationName;
+
+  /**
+   * Constructs an AbstractAnnotatedClassDiscovery object.
+   *
+   * @param string $plugin_definition_annotation_name
+   *   (optional) The name of the annotation that contains the plugin definition.
+   *   Defaults to 'Drupal\Component\Annotation\Plugin'.
+   */
+  function __construct($plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+    $this->pluginDefinitionAnnotationName = $plugin_definition_annotation_name;
+  }
+
+  /**
+   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinition().
+   */
+  public function getDefinition($plugin_id) {
+    $plugins = $this->getDefinitions();
+    return isset($plugins[$plugin_id]) ? $plugins[$plugin_id] : NULL;
+  }
+
+  /**
+   * @param $class
+   * @return bool
+   */
+  public function loadAnnotationClass($class) {
+
+    if (class_exists($class, FALSE)) {
+      return TRUE;
+    }
+
+    foreach ($this->getAnnotationNamespaces() as $namespace => $dirs) {
+      if (0 === strpos($class, $namespace)) {
+        if (TRUE === $dirs) {
+          // Use the regular class loader.
+          return class_exists($class);
+        }
+        // Treat $dirs as PSR-4 directories.
+        $relativePath = str_replace('\\', '/', substr($class, strlen($namespace))) . '.php';
+        foreach ((array) $dirs as $dir) {
+          if (file_exists($file = $dir . '/' . $relativePath)) {
+            require $file;
+            return TRUE;
+          }
+        }
+      }
+    }
+
+    return FALSE;
+  }
+
+  /**
+   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
+   */
+  public function getDefinitions() {
+    $definitions = array();
+    $reader = new AnnotationReader();
+    // Prevent @endlink from being parsed as an annotation.
+    $reader->addGlobalIgnoredName('endlink');
+    $reader->addGlobalIgnoredName('file');
+
+    // Register the namespaces of classes that can be used for annotations.
+    AnnotationRegistry::registerLoader(array($this, 'loadAnnotationClass'));
+
+    // Search for classes within all PSR-0 namespace locations.
+    foreach ($this->getPluginNamespaces() as $namespace => $dirs) {
+      foreach ($dirs as $dir) {
+        if (file_exists($dir)) {
+          /**
+           * @var DirectoryIterator $fileinfo
+           */
+          foreach (new DirectoryIterator($dir) as $fileinfo) {
+            // @todo Once core requires 5.3.6, use $fileinfo->getExtension().
+            if (pathinfo($fileinfo->getFilename(), PATHINFO_EXTENSION) == 'php') {
+              $class = $namespace . '\\' . $fileinfo->getBasename('.php');
+
+              // The filename is already known, so there is no need to find the
+              // file. However, StaticReflectionParser needs a finder, so use a
+              // mock version.
+              $finder = MockFileFinder::create($fileinfo->getPathName());
+              $parser = new StaticReflectionParser($class, $finder);
+
+              if ($annotation = $reader->getClassAnnotation($parser->getReflectionClass(), $this->pluginDefinitionAnnotationName)) {
+                // AnnotationInterface::get() returns the array definition
+                // instead of requiring us to work with the annotation object.
+                $definition = $annotation->get();
+                $definition['class'] = $class;
+                $definitions[$definition['id']] = $definition;
+              }
+            }
+          }
+        }
+      }
+    }
+
+    // Don't let the loaders pile up.
+    AnnotationRegistry::reset();
+
+    return $definitions;
+  }
+
+  /**
+   * Returns an array of PSR-0 namespaces to search for plugin classes.
+   */
+  protected abstract function getPluginNamespaces();
+
+  /**
+   * Returns an array of PSR-0 namespaces to search for annotation classes.
+   */
+  protected abstract function getAnnotationNamespaces();
+
+}
diff --git a/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
index 13f853d..cf33fb8 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -7,17 +7,13 @@
 
 namespace Drupal\Component\Plugin\Discovery;
 
-use DirectoryIterator;
-use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
-use Drupal\Component\Reflection\MockFileFinder;
 use Doctrine\Common\Annotations\AnnotationReader;
 use Doctrine\Common\Annotations\AnnotationRegistry;
-use Doctrine\Common\Reflection\StaticReflectionParser;
 
 /**
  * Defines a discovery mechanism to find annotated plugins in PSR-0 namespaces.
  */
-class AnnotatedClassDiscovery implements DiscoveryInterface {
+class AnnotatedClassDiscovery extends AbstractAnnotatedClassDiscovery {
 
   /**
    * The namespaces within which to find plugin classes.
@@ -34,16 +30,6 @@ class AnnotatedClassDiscovery implements DiscoveryInterface {
   protected $annotationNamespaces;
 
   /**
-   * The name of the annotation that contains the plugin definition.
-   *
-   * The class corresponding to this name must implement
-   * \Drupal\Component\Annotation\AnnotationInterface.
-   *
-   * @var string
-   */
-  protected $pluginDefinitionAnnotationName;
-
-  /**
    * Constructs an AnnotatedClassDiscovery object.
    *
    * @param array $plugin_namespaces
@@ -59,70 +45,18 @@ class AnnotatedClassDiscovery implements DiscoveryInterface {
   function __construct($plugin_namespaces = array(), $annotation_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
     $this->pluginNamespaces = $plugin_namespaces;
     $this->annotationNamespaces = $annotation_namespaces;
-    $this->pluginDefinitionAnnotationName = $plugin_definition_annotation_name;
-  }
-
-  /**
-   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinition().
-   */
-  public function getDefinition($plugin_id) {
-    $plugins = $this->getDefinitions();
-    return isset($plugins[$plugin_id]) ? $plugins[$plugin_id] : NULL;
-  }
-
-  /**
-   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
-   */
-  public function getDefinitions() {
-    $definitions = array();
-    $reader = new AnnotationReader();
-    // Prevent @endlink from being parsed as an annotation.
-    $reader->addGlobalIgnoredName('endlink');
-    $reader->addGlobalIgnoredName('file');
-
-    // Register the namespaces of classes that can be used for annotations.
-    AnnotationRegistry::registerAutoloadNamespaces($this->getAnnotationNamespaces());
-
-    // Search for classes within all PSR-0 namespace locations.
-    foreach ($this->getPluginNamespaces() as $namespace => $dirs) {
-      foreach ($dirs as $dir) {
-        $dir .= DIRECTORY_SEPARATOR . str_replace('\\', DIRECTORY_SEPARATOR, $namespace);
-        if (file_exists($dir)) {
-          foreach (new DirectoryIterator($dir) as $fileinfo) {
-            // @todo Once core requires 5.3.6, use $fileinfo->getExtension().
-            if (pathinfo($fileinfo->getFilename(), PATHINFO_EXTENSION) == 'php') {
-              $class = $namespace . '\\' . $fileinfo->getBasename('.php');
-
-              // The filename is already known, so there is no need to find the
-              // file. However, StaticReflectionParser needs a finder, so use a
-              // mock version.
-              $finder = MockFileFinder::create($fileinfo->getPathName());
-              $parser = new StaticReflectionParser($class, $finder);
-
-              if ($annotation = $reader->getClassAnnotation($parser->getReflectionClass(), $this->pluginDefinitionAnnotationName)) {
-                // AnnotationInterface::get() returns the array definition
-                // instead of requiring us to work with the annotation object.
-                $definition = $annotation->get();
-                $definition['class'] = $class;
-                $definitions[$definition['id']] = $definition;
-              }
-            }
-          }
-        }
-      }
-    }
-    return $definitions;
+    parent::__construct($plugin_definition_annotation_name);
   }
 
   /**
-   * Returns an array of PSR-0 namespaces to search for plugin classes.
+   * @inheritdoc
    */
   protected function getPluginNamespaces() {
     return $this->pluginNamespaces;
   }
 
   /**
-   * Returns an array of PSR-0 namespaces to search for annotation classes.
+   * @inheritdoc
    */
   protected function getAnnotationNamespaces() {
     return $this->annotationNamespaces;
diff --git a/core/lib/Drupal/Core/Action/ActionManager.php b/core/lib/Drupal/Core/Action/ActionManager.php
index a3bfeda..8013724 100644
--- a/core/lib/Drupal/Core/Action/ActionManager.php
+++ b/core/lib/Drupal/Core/Action/ActionManager.php
@@ -28,7 +28,7 @@ class ActionManager extends PluginManagerBase {
    *   keyed by the corresponding namespace to look for plugin implementations.
    */
   public function __construct(\Traversable $namespaces) {
-    $this->discovery = new AnnotatedClassDiscovery('Plugin/Action', $namespaces, array(), 'Drupal\Core\Annotation\Action');
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\Action', 'Drupal\Core\Annotation\Action');
     $this->discovery = new AlterDecorator($this->discovery, 'action_info');
 
     $this->factory = new ContainerFactory($this);
diff --git a/core/lib/Drupal/Core/Archiver/ArchiverManager.php b/core/lib/Drupal/Core/Archiver/ArchiverManager.php
index 14c2a9c..78d431f 100644
--- a/core/lib/Drupal/Core/Archiver/ArchiverManager.php
+++ b/core/lib/Drupal/Core/Archiver/ArchiverManager.php
@@ -31,7 +31,7 @@ class ArchiverManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Plugin/Archiver', $namespaces);
+    parent::__construct($namespaces, 'Plugin\Archiver');
     $this->alterInfo($module_handler, 'archiver_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'archiver_info');
   }
diff --git a/core/lib/Drupal/Core/Autoload/ClassLoader.php b/core/lib/Drupal/Core/Autoload/ClassLoader.php
new file mode 100644
index 0000000..1e1e1f5
--- /dev/null
+++ b/core/lib/Drupal/Core/Autoload/ClassLoader.php
@@ -0,0 +1,372 @@
+<?php
+
+namespace Drupal\Core\Autoload;
+
+/**
+ * ClassLoader implements a PSR-0 class loader
+ *
+ * See https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md
+ *
+ *     $loader = new \Composer\Autoload\ClassLoader();
+ *
+ *     // register classes with namespaces
+ *     $loader->add('Symfony\Component', __DIR__.'/component');
+ *     $loader->add('Symfony',           __DIR__.'/framework');
+ *
+ *     // activate the autoloader
+ *     $loader->register();
+ *
+ *     // to enable searching the include path (eg. for PEAR packages)
+ *     $loader->setUseIncludePath(true);
+ *
+ * In this example, if you try to use a class in the Symfony\Component
+ * namespace or one of its children (Symfony\Component\Console for instance),
+ * the autoloader will first look for the class under the component/
+ * directory, and it will then fallback to the framework/ directory if not
+ * found before giving up.
+ *
+ * This class is loosely based on the Symfony UniversalClassLoader.
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ * @author Jordi Boggiano <j.boggiano@seld.be>
+ */
+class ClassLoader {
+
+  // PSR-4
+  private $prefixLengthsPsr4 = array();
+  private $prefixDirsPsr4 = array();
+  private $fallbackDirsPsr4 = array();
+
+  // PSR-0
+  private $prefixesPsr0 = array();
+  private $fallbackDirsPsr0 = array();
+
+  private $useIncludePath = false;
+  private $classMap = array();
+
+  const PREDICTOR_INDEX = 9;
+
+  public function getPrefixes() {
+    return call_user_func_array('array_merge', $this->prefixesPsr0);
+  }
+
+  public function getPrefixesPsr4() {
+    return $this->prefixDirsPsr4;
+  }
+
+  public function getFallbackDirs() {
+    return $this->fallbackDirsPsr0;
+  }
+
+  public function getFallbackDirsPsr4() {
+    return $this->fallbackDirsPsr4;
+  }
+
+  public function getClassMap() {
+    return $this->classMap;
+  }
+
+  /**
+   * @param array $classMap Class to filename map
+   */
+  public function addClassMap(array $classMap) {
+    if ($this->classMap) {
+      $this->classMap = array_merge($this->classMap, $classMap);
+    }
+    else {
+      $this->classMap = $classMap;
+    }
+  }
+
+  /**
+   * Registers a set of PSR-0 directories for a given prefix, either
+   * appending or prepending to the ones previously set for this prefix.
+   *
+   * @param string       $prefix  The prefix
+   * @param array|string $paths   The PSR-0 root directories
+   * @param bool         $prepend Whether to prepend the directories
+   */
+  public function add($prefix, $paths, $prepend = false) {
+    if (!$prefix) {
+      if ($prepend) {
+        $this->fallbackDirsPsr0 = array_merge(
+          (array) $paths,
+          $this->fallbackDirsPsr0
+        );
+      }
+      else {
+        $this->fallbackDirsPsr0 = array_merge(
+          $this->fallbackDirsPsr0,
+          (array) $paths
+        );
+      }
+
+      return;
+    }
+
+    $first = $prefix[0];
+    if (!isset($this->prefixesPsr0[$first][$prefix])) {
+      $this->prefixesPsr0[$first][$prefix] = (array) $paths;
+
+      return;
+    }
+    if ($prepend) {
+      $this->prefixesPsr0[$first][$prefix] = array_merge(
+        (array) $paths,
+        $this->prefixesPsr0[$first][$prefix]
+      );
+    }
+    else {
+      $this->prefixesPsr0[$first][$prefix] = array_merge(
+        $this->prefixesPsr0[$first][$prefix],
+        (array) $paths
+      );
+    }
+  }
+
+  /**
+   * Registers a set of PSR-4 directories for a given namespace, either
+   * appending or prepending to the ones previously set for this namespace.
+   *
+   * @param string $prefix  The prefix/namespace, with trailing '\\'
+   * @param array|string $paths   The PSR-0 base directories
+   * @param bool $prepend Whether to prepend the directories
+   * @throws \Exception
+   */
+  public function addPsr4($prefix, $paths, $prepend = false) {
+    if (!$prefix) {
+      // Register directories for the root namespace.
+      if ($prepend) {
+        $this->fallbackDirsPsr4 = array_merge(
+          (array) $paths,
+          $this->fallbackDirsPsr4
+        );
+      }
+      else {
+        $this->fallbackDirsPsr4 = array_merge(
+          $this->fallbackDirsPsr4,
+          (array) $paths
+        );
+      }
+    }
+    elseif (!isset($this->prefixDirsPsr4[$prefix])) {
+      // Register directories for a new namespace.
+      $length = strlen($prefix);
+      if ('\\' !== $prefix[$length - 1]) {
+        throw new \Exception("A non-empty PSR-4 prefix must end with a namespace separator.");
+      }
+      if ($length > self::PREDICTOR_INDEX) {
+        $predictor = $prefix[0] . $prefix[self::PREDICTOR_INDEX];
+        $this->prefixLengthsPsr4[$predictor][$prefix] = $length;
+      }
+      else {
+        $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
+      }
+      $this->prefixDirsPsr4[$prefix] = (array) $paths;
+    }
+    elseif ($prepend) {
+      // Prepend directories for an already registered namespace.
+      $this->prefixDirsPsr4[$prefix] = array_merge(
+        (array) $paths,
+        $this->prefixDirsPsr4[$prefix]
+      );
+    }
+    else {
+      // Append directories for an already registered namespace.
+      $this->prefixDirsPsr4[$prefix] = array_merge(
+        $this->prefixDirsPsr4[$prefix],
+        (array) $paths
+      );
+    }
+  }
+
+  /**
+   * Registers a set of PSR-0 directories for a given prefix,
+   * replacing any others previously set for this prefix.
+   *
+   * @param string       $prefix The prefix
+   * @param array|string $paths  The PSR-0 base directories
+   */
+  public function set($prefix, $paths) {
+    if (!$prefix) {
+      $this->fallbackDirsPsr0 = (array) $paths;
+    }
+    else {
+      $this->prefixesPsr0[$prefix[0]][$prefix] = (array) $paths;
+    }
+  }
+
+  /**
+   * Registers a set of PSR-4 directories for a given namespace,
+   * replacing any others previously set for this namespace.
+   *
+   * @param string $prefix  The prefix/namespace, with trailing '\\'
+   * @param array|string $paths   The PSR-4 base directories
+   * @throws \Exception
+   */
+  public function setPsr4($prefix, $paths) {
+    if (!$prefix) {
+      $this->fallbackDirsPsr4 = (array) $paths;
+    }
+    else {
+      $length = strlen($prefix);
+      if ('\\' !== $prefix[$length - 1]) {
+        throw new \Exception("A non-empty PSR-4 prefix must end with a namespace separator.");
+      }
+      if ($length > self::PREDICTOR_INDEX) {
+        $predictor = $prefix[0] . $prefix[self::PREDICTOR_INDEX];
+        $this->prefixLengthsPsr4[$predictor][$prefix] = $length;
+      }
+      else {
+        $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
+      }
+      $this->prefixDirsPsr4[$prefix] = (array) $paths;
+    }
+  }
+
+  /**
+   * Turns on searching the include path for class files.
+   *
+   * @param bool $useIncludePath
+   */
+  public function setUseIncludePath($useIncludePath) {
+    $this->useIncludePath = $useIncludePath;
+  }
+
+  /**
+   * Can be used to check if the autoloader uses the include path to check
+   * for classes.
+   *
+   * @return bool
+   */
+  public function getUseIncludePath() {
+    return $this->useIncludePath;
+  }
+
+  /**
+   * Registers this instance as an autoloader.
+   *
+   * @param bool $prepend Whether to prepend the autoloader or not
+   */
+  public function register($prepend = false) {
+    spl_autoload_register(array($this, 'loadClass'), true, $prepend);
+  }
+
+  /**
+   * Unregisters this instance as an autoloader.
+   */
+  public function unregister() {
+    spl_autoload_unregister(array($this, 'loadClass'));
+  }
+
+  /**
+   * Loads the given class or interface.
+   *
+   * @param  string    $class The name of the class
+   * @return bool|null True if loaded, null otherwise
+   */
+  public function loadClass($class) {
+    if ($file = $this->findFile($class)) {
+      include $file;
+
+      return true;
+    }
+  }
+
+  /**
+   * Finds the path to the file where the class is defined.
+   *
+   * @param string $class The name of the class
+   *
+   * @return string|false The path if found, false otherwise
+   */
+  public function findFile($class) {
+    // work around for PHP 5.3.0 - 5.3.2 https://bugs.php.net/50731
+    if ('\\' == $class[0]) {
+      $class = substr($class, 1);
+    }
+
+    // class map lookup
+    if (isset($this->classMap[$class])) {
+      return $this->classMap[$class];
+    }
+
+    // PSR-4 lookup
+    $logicalPathPsr4 = strtr($class, '\\', DIRECTORY_SEPARATOR) . '.php';
+
+    $first = $class[0];
+    if (isset($class[self::PREDICTOR_INDEX])) {
+      $predictor = $first . $class[self::PREDICTOR_INDEX];
+      if (isset($this->prefixLengthsPsr4[$predictor])) {
+        foreach ($this->prefixLengthsPsr4[$predictor] as $prefix => $length) {
+          if (0 === strpos($class, $prefix)) {
+            foreach ($this->prefixDirsPsr4[$prefix] as $dir) {
+              if (file_exists($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
+                return $file;
+              }
+            }
+          }
+        }
+      }
+    }
+
+    if (isset($this->prefixLengthsPsr4[$first])) {
+      foreach ($this->prefixLengthsPsr4[$first] as $prefix => $length) {
+        if (0 === strpos($class, $prefix)) {
+          foreach ($this->prefixDirsPsr4[$prefix] as $dir) {
+            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . substr($logicalPathPsr4, $length))) {
+              return $file;
+            }
+          }
+        }
+      }
+    }
+
+    // PSR-4 fallback dirs
+    foreach ($this->fallbackDirsPsr4 as $dir) {
+      if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr4)) {
+        return $file;
+      }
+    }
+
+    // PSR-0 lookup
+    if (false !== $pos = strrpos($class, '\\')) {
+      // namespaced class name
+      $logicalPathPsr0
+        = substr($logicalPathPsr4, 0, $pos + 1)
+        . strtr(substr($logicalPathPsr4, $pos + 1), '_', DIRECTORY_SEPARATOR)
+      ;
+    }
+    else {
+      // PEAR-like class name
+      $logicalPathPsr0 = strtr($class, '_', DIRECTORY_SEPARATOR) . '.php';
+    }
+
+    if (isset($this->prefixesPsr0[$first])) {
+      foreach ($this->prefixesPsr0[$first] as $prefix => $dirs) {
+        if (0 === strpos($class, $prefix)) {
+          foreach ($dirs as $dir) {
+            if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
+              return $file;
+            }
+          }
+        }
+      }
+    }
+
+    // PSR-0 fallback dirs
+    foreach ($this->fallbackDirsPsr0 as $dir) {
+      if (file_exists($file = $dir . DIRECTORY_SEPARATOR . $logicalPathPsr0)) {
+        return $file;
+      }
+    }
+
+    // PSR-0 include paths.
+    if ($this->useIncludePath && $file = stream_resolve_include_path($logicalPathPsr0)) {
+      return $file;
+    }
+
+    // Remember that this class does not exist.
+    return $this->classMap[$class] = false;
+  }
+}
\ No newline at end of file
diff --git a/core/lib/Drupal/Core/Autoload/DrupalAutoloaderInit.php b/core/lib/Drupal/Core/Autoload/DrupalAutoloaderInit.php
new file mode 100644
index 0000000..204ec52
--- /dev/null
+++ b/core/lib/Drupal/Core/Autoload/DrupalAutoloaderInit.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace Drupal\Core\Autoload;
+
+class DrupalAutoloaderInit
+{
+  private static $loader;
+
+  public static function loadClassLoader($class) {
+    if ('Drupal\Core\Autoload\ClassLoader' === $class) {
+      require __DIR__ . '/ClassLoader.php';
+    }
+  }
+
+  public static function getLoader() {
+    if (null !== self::$loader) {
+      return self::$loader;
+    }
+
+    spl_autoload_register(array('Drupal\Core\Autoload\DrupalAutoloaderInit', 'loadClassLoader'), true, true);
+    self::$loader = $loader = new ClassLoader();
+    spl_autoload_unregister(array('Drupal\Core\Autoload\DrupalAutoloaderInit', 'loadClassLoader'));
+
+    $coreDir = dirname(dirname(dirname(dirname(__DIR__))));
+    $vendorDir = $coreDir . '/vendor';
+    $composerDir = $vendorDir . '/composer';
+    $baseDir = dirname($coreDir);
+
+    $includePaths = require $composerDir . '/include_paths.php';
+    array_push($includePaths, get_include_path());
+    set_include_path(join(PATH_SEPARATOR, $includePaths));
+
+    $map = require $composerDir . '/autoload_namespaces.php';
+    unset($map['Drupal\Core']);
+    unset($map['Drupal\Component']);
+    unset($map['Drupal\Driver']);
+    $loader->setPsr4('Drupal\Core\\', $baseDir . '/core/lib/Drupal/Core');
+    $loader->setPsr4('Drupal\Component\\', $baseDir . '/core/lib/Drupal/Component');
+    $loader->setPsr4('Drupal\Driver\\', $baseDir . '/core/lib/Drupal/Driver');
+    foreach ($map as $namespace => $path) {
+      $loader->set($namespace, $path);
+    }
+
+    $classMap = require $composerDir . '/autoload_classmap.php';
+    if ($classMap) {
+      $loader->addClassMap($classMap);
+    }
+
+    $loader->register(true);
+
+    // @todo This can be updated once Composer provides a autoload_files.php.
+    require $vendorDir . '/kriswallsmith/assetic/src/functions.php';
+    require $baseDir . '/core/lib/Drupal.php';
+
+    return $loader;
+  }
+}
diff --git a/core/lib/Drupal/Core/Condition/ConditionManager.php b/core/lib/Drupal/Core/Condition/ConditionManager.php
index 7f5d630..090ee9a 100644
--- a/core/lib/Drupal/Core/Condition/ConditionManager.php
+++ b/core/lib/Drupal/Core/Condition/ConditionManager.php
@@ -36,10 +36,8 @@ public function __construct(\Traversable $namespaces, CacheBackendInterface $cac
     $this->alterInfo($module_handler, 'condition_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'condition');
 
-    $annotation_namespaces = array(
-      'Drupal\Core\Condition\Annotation' => DRUPAL_ROOT . '/core/lib',
-    );
-    parent::__construct('Plugin/Condition', $namespaces, $annotation_namespaces, 'Drupal\Core\Condition\Annotation\Condition');
+    parent::__construct($namespaces, 'Plugin\Condition', 'Drupal\Core\Condition\Annotation\Condition');
+    $this->addAnnotationNamespace('Drupal\Core\Condition\Annotation');
   }
 
   /**
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 1bf9c05..35c18ae 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -19,7 +19,7 @@
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\HttpKernel\TerminableInterface;
-use Composer\Autoload\ClassLoader;
+use Drupal\Core\Autoload\ClassLoader;
 
 /**
  * The DrupalKernel class is the core of Drupal itself.
@@ -97,7 +97,7 @@ class DrupalKernel implements DrupalKernelInterface, TerminableInterface {
   /**
    * The classloader object.
    *
-   * @var \Composer\Autoload\ClassLoader
+   * @var \Drupal\Core\Autoload\ClassLoader
    */
   protected $classLoader;
 
@@ -150,7 +150,7 @@ class DrupalKernel implements DrupalKernelInterface, TerminableInterface {
    *   String indicating the environment, e.g. 'prod' or 'dev'. Used by
    *   Symfony\Component\HttpKernel\Kernel::__construct(). Drupal does not use
    *   this value currently. Pass 'prod'.
-   * @param \Composer\Autoload\ClassLoader $class_loader
+   * @param \Drupal\Core\Autoload\ClassLoader $class_loader
    *   (optional) The classloader is only used if $storage is not given or
    *   the load from storage fails and a container rebuild is required. In
    *   this case, the loaded modules will be registered with this loader in
@@ -218,7 +218,7 @@ public function discoverServiceProviders() {
       $this->moduleList = isset($module_list['enabled']) ? $module_list['enabled'] : array();
     }
     $module_filenames = $this->getModuleFileNames();
-    $this->registerNamespaces($this->getModuleNamespaces($module_filenames));
+    $this->registerNamespacesPsr4($this->getModuleNamespacesPsr4($module_filenames));
 
     // Load each module's serviceProvider class.
     foreach ($this->moduleList as $module => $weight) {
@@ -402,8 +402,8 @@ protected function initializeContainer() {
       // All namespaces must be registered before we attempt to use any service
       // from the container.
       $container_modules = $this->container->getParameter('container.modules');
-      $namespaces_before = $this->classLoader->getPrefixes();
-      $this->registerNamespaces($this->getModuleNamespaces($container_modules));
+      $namespaces_before = $this->classLoader->getPrefixesPsr4();
+      $this->registerNamespacesPsr4($this->getModuleNamespacesPsr4($container_modules));
 
       // If 'container.modules' is wrong, the container must be rebuilt.
       if (!isset($this->moduleList)) {
@@ -416,9 +416,9 @@ protected function initializeContainer() {
         // registerNamespaces() performs a merge rather than replace, so to
         // effectively remove erroneous registrations, we must replace them with
         // empty arrays.
-        $namespaces_after = $this->classLoader->getPrefixes();
+        $namespaces_after = $this->classLoader->getPrefixesPsr4();
         $namespaces_before += array_fill_keys(array_diff(array_keys($namespaces_after), array_keys($namespaces_before)), array());
-        $this->registerNamespaces($namespaces_before);
+        $this->registerNamespacesPsr4($namespaces_before);
       }
     }
 
@@ -492,14 +492,18 @@ protected function buildContainer() {
     $container->setParameter('container.modules', $this->getModuleFileNames());
 
     // Get a list of namespaces and put it onto the container.
-    $namespaces = $this->getModuleNamespaces($this->getModuleFileNames());
+    $namespaces = $this->getModuleNamespacesPsr4($this->getModuleFileNames());
     // Add all components in \Drupal\Core and \Drupal\Component that have a
     // Plugin directory.
     foreach (array('Core', 'Component') as $parent_directory) {
       $path = DRUPAL_ROOT . '/core/lib/Drupal/' . $parent_directory;
+      $parent_namespace = 'Drupal\\' . $parent_directory;
+      /**
+       * @var \DirectoryIterator $component
+       */
       foreach (new \DirectoryIterator($path) as $component) {
         if (!$component->isDot() && is_dir($component->getPathname() . '/Plugin')) {
-          $namespaces['Drupal\\' . $parent_directory  .'\\' . $component->getFilename()] = DRUPAL_ROOT . '/core/lib';
+          $namespaces[$parent_namespace . '\\' . $component->getFilename()] = $path . '/' . $component->getFilename();
         }
       }
     }
@@ -632,7 +636,30 @@ protected function getModuleFileNames() {
   }
 
   /**
-   * Gets the namespaces of each enabled module.
+   * Gets the namespaces of each enabled module,
+   * each with their PSR-4 directories.
+   *
+   * @param array $moduleFileNames
+   * @return array
+   */
+  protected function getModuleNamespacesPsr4($moduleFileNames) {
+    $namespaces = array();
+    foreach ($moduleFileNames as $module => $filename) {
+      $namespaces["Drupal\\$module"][] = DRUPAL_ROOT . '/' . dirname($filename) . '/lib/Drupal/' . $module;
+      // For the time being, register lib and src likewise,
+      // until this is decided in https://drupal.org/node/1971198
+      $namespaces["Drupal\\$module"][] = DRUPAL_ROOT . '/' . dirname($filename) . '/lib';
+      $namespaces["Drupal\\$module"][] = DRUPAL_ROOT . '/' . dirname($filename) . '/src';
+    }
+    return $namespaces;
+  }
+
+  /**
+   * Gets the namespaces of each enabled module,
+   * each with its PSR-0 directory.
+   *
+   * @param array $moduleFileNames
+   * @return array
    */
   protected function getModuleNamespaces($moduleFileNames) {
     $namespaces = array();
@@ -643,7 +670,20 @@ protected function getModuleNamespaces($moduleFileNames) {
   }
 
   /**
-   * Registers a list of namespaces.
+   * Registers a list of namespaces with PSR-4 directories.
+   *
+   * @param array $namespaces
+   */
+  protected function registerNamespacesPsr4(array $namespaces = array()) {
+    foreach ($namespaces as $prefix => $paths) {
+      $this->classLoader->addPsr4($prefix . '\\', $paths);
+    }
+  }
+
+  /**
+   * Registers a list of namespaces with PSR-0 directories.
+   *
+   * @param array $namespaces
    */
   protected function registerNamespaces(array $namespaces = array()) {
     foreach ($namespaces as $prefix => $path) {
diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index 3060909..5939ab4 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -136,11 +136,11 @@ public function __construct(\Traversable $namespaces, ContainerInterface $contai
   }
 
   protected function doDiscovery($namespaces) {
-    $annotation_namespaces = array(
-      'Drupal\Core\Entity\Annotation' => DRUPAL_ROOT . '/core/lib',
-    );
-    $this->discovery = new AnnotatedClassDiscovery('Entity', $namespaces, $annotation_namespaces, 'Drupal\Core\Entity\Annotation\EntityType');
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Entity', 'Drupal\Core\Entity\Annotation\EntityType');
+    $this->discovery->addAnnotationNamespace('Drupal\Core\Entity\Annotation');
     $this->discovery = new InfoHookDecorator($this->discovery, 'entity_info');
+
+    // Allow the plugin definition to be altered by hook_entity_info_alter().
     $this->discovery = new AlterDecorator($this->discovery, 'entity_info');
     $this->discovery = new CacheDecorator($this->discovery, 'entity_info:' . $this->languageManager->getLanguage(Language::TYPE_INTERFACE)->id, 'cache', CacheBackendInterface::CACHE_PERMANENT, array('entity_info' => TRUE));
   }
@@ -192,6 +192,7 @@ public function hasController($entity_type, $controller_type) {
    *   (optional) If this controller definition is nested, the name of the key.
    *   Defaults to NULL.
    *
+   * @throws \InvalidArgumentException
    * @return string
    *   The class name for this controller instance.
    */
diff --git a/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php b/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php
index 0be215e..6a60cc0 100644
--- a/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php
+++ b/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php
@@ -42,10 +42,8 @@ class FieldTypePluginManager extends DefaultPluginManager {
    *   The module handler.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array(
-      'Drupal\Core\Entity\Annotation' => DRUPAL_ROOT . '/core/lib',
-    );
-    parent::__construct('Plugin/field/field_type', $namespaces, $annotation_namespaces, 'Drupal\Core\Entity\Annotation\FieldType');
+    parent::__construct($namespaces, 'Plugin\field\field_type', 'Drupal\Core\Entity\Annotation\FieldType');
+    $this->addAnnotationNamespace('Drupal\Core\Entity\Annotation');
     $this->alterInfo($module_handler, 'field_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'field_types');
 
diff --git a/core/lib/Drupal/Core/Menu/LocalActionManager.php b/core/lib/Drupal/Core/Menu/LocalActionManager.php
index c99dd83..adf41db 100644
--- a/core/lib/Drupal/Core/Menu/LocalActionManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalActionManager.php
@@ -65,7 +65,7 @@ class LocalActionManager extends DefaultPluginManager {
    *   The language manager.
    */
   public function __construct(\Traversable $namespaces, ControllerResolverInterface $controller_resolver, Request $request, ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend, LanguageManager $language_manager) {
-    parent::__construct('Plugin/Menu/LocalAction', $namespaces, array(), 'Drupal\Core\Annotation\Menu\LocalAction');
+    parent::__construct($namespaces, 'Plugin\Menu\LocalAction', 'Drupal\Core\Annotation\Menu\LocalAction');
 
     $this->controllerResolver = $controller_resolver;
     $this->request = $request;
diff --git a/core/lib/Drupal/Core/Menu/LocalTaskManager.php b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
index 17b5128..9b1bb16 100644
--- a/core/lib/Drupal/Core/Menu/LocalTaskManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
@@ -73,7 +73,7 @@ class LocalTaskManager extends DefaultPluginManager {
    *   The language manager.
    */
   public function __construct(\Traversable $namespaces, ControllerResolverInterface $controller_resolver, Request $request, RouteProviderInterface $route_provider, ModuleHandlerInterface $module_handler, CacheBackendInterface $cache, LanguageManager $language_manager) {
-    parent::__construct('Plugin/Menu/LocalTask', $namespaces, array(), 'Drupal\Core\Annotation\Menu\LocalTask');
+    parent::__construct($namespaces, 'Plugin\Menu\LocalTask', 'Drupal\Core\Annotation\Menu\LocalTask');
     $this->controllerResolver = $controller_resolver;
     $this->request = $request;
     $this->routeProvider = $route_provider;
diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index f802b0c..fec33e0 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -25,6 +25,11 @@
 class DefaultPluginManager extends PluginManagerBase implements PluginManagerInterface, CachedDiscoveryInterface {
 
   /**
+   * @var \Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery
+   */
+  protected $originalDiscovery;
+
+  /**
    * Cached definitions array.
    *
    * @var array
@@ -91,26 +96,33 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
   /**
    * Creates the discovery object.
    *
-   * @param string|bool $subdir
-   *   The plugin's subdirectory, for example Plugin/views/filter.
    * @param \Traversable $namespaces
    *   An object that implements \Traversable which contains the root paths
    *   keyed by the corresponding namespace to look for plugin implementations.
-   * @param array $annotation_namespaces
-   *   (optional) The namespaces of classes that can be used as annotations.
-   *   Defaults to an empty array.
+   * @param string|bool $namespace_suffix
+   *   The suffix to append to each of the root namespaces, to obtain the plugin
+   *   namespaces.
    * @param string $plugin_definition_annotation_name
    *   (optional) The name of the annotation that contains the plugin definition.
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
    */
-  public function __construct($subdir, \Traversable $namespaces, $annotation_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
-    $this->subdir = $subdir;
-    $this->discovery = new AnnotatedClassDiscovery($subdir, $namespaces, $annotation_namespaces, $plugin_definition_annotation_name);
-    $this->discovery = new ContainerDerivativeDiscoveryDecorator($this->discovery);
+  public function __construct(\Traversable $namespaces, $namespace_suffix, $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
+    $this->originalDiscovery = new AnnotatedClassDiscovery($namespaces, $namespace_suffix, $plugin_definition_annotation_name);
+    $this->discovery = new ContainerDerivativeDiscoveryDecorator($this->originalDiscovery);
     $this->factory = new ContainerFactory($this);
   }
 
   /**
+   * @param string $namespace
+   * @param bool|string|array $dir
+   *   The PSR-4 directory, or an array of PSR-4 directories, or
+   *   TRUE, to use the regular class loader via class_exists().
+   */
+  protected function addAnnotationNamespace($namespace, $dir = TRUE) {
+    $this->originalDiscovery->addAnnotationNamespace($namespace, $dir);
+  }
+
+  /**
    * Initialize the cache backend.
    *
    * Plugin definitions are cached using the provided cache backend. The
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
index 63c87ab..c283226 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -7,12 +7,19 @@
 
 namespace Drupal\Core\Plugin\Discovery;
 
-use Drupal\Component\Plugin\Discovery\AnnotatedClassDiscovery as ComponentAnnotatedClassDiscovery;
+use Drupal\Component\Plugin\Discovery\AbstractAnnotatedClassDiscovery;
 
 /**
  * Defines a discovery mechanism to find annotated plugins in PSR-0 namespaces.
  */
-class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
+class AnnotatedClassDiscovery extends AbstractAnnotatedClassDiscovery {
+
+  /**
+   * An object containing the namespaces to look for plugin implementations.
+   *
+   * @var \Traversable
+   */
+  protected $rootNamespacesIterator;
 
   /**
    * The subdirectory within a namespace to look for plugins.
@@ -22,43 +29,63 @@ class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
    *
    * @var string
    */
-  protected $subdir = '';
+  protected $directorySuffix = '';
 
   /**
-   * An object containing the namespaces to look for plugin implementations.
+   * The subdirectory within a namespace to look for plugins.
    *
-   * @var \Traversable
+   * If the plugins are in the top level of the namespace and not within a
+   * subdirectory, set this to an empty string.
+   *
+   * @var string
    */
-  protected $rootNamespacesIterator;
+  protected $namespaceSuffix = '';
+
+  /**
+   * The namespaces of classes that can be used as annotations.
+   *
+   * @var array
+   */
+  protected $annotationNamespaces = array();
 
   /**
    * Constructs an AnnotatedClassDiscovery object.
    *
-   * @param string $subdir
-   *   Either the plugin's subdirectory, for example 'Plugin/views/filter', or
-   *   empty string if plugins are located at the top level of the namespace.
    * @param \Traversable $root_namespaces
    *   An object that implements \Traversable which contains the root paths
    *   keyed by the corresponding namespace to look for plugin implementations.
    *   If $subdir is not an empty string, it will be appended to each namespace.
-   * @param array $annotation_namespaces
-   *   (optional) The namespaces of classes that can be used as annotations.
-   *   Defaults to an empty array.
+   * @param string $namespace_suffix
+   *   Suffix to append to each of the root namespaces, to obtain the plugin
+   *   namespaces. E.g. '\Plugin\views\filter', or empty string if plugins are
+   *   located at the top level of each of the root namespaces.
    * @param string $plugin_definition_annotation_name
    *   (optional) The name of the annotation that contains the plugin definition.
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
    */
-  function __construct($subdir, \Traversable $root_namespaces, $annotation_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
-    if ($subdir) {
-      $this->subdir = str_replace('/', '\\', $subdir);
-    }
+  public function __construct(\Traversable $root_namespaces, $namespace_suffix = '', $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
     $this->rootNamespacesIterator = $root_namespaces;
-    $annotation_namespaces += array(
-      'Drupal\Component\Annotation' => DRUPAL_ROOT . '/core/lib',
-      'Drupal\Core\Annotation' => DRUPAL_ROOT . '/core/lib',
-    );
-    $plugin_namespaces = array();
-    parent::__construct($plugin_namespaces, $annotation_namespaces, $plugin_definition_annotation_name);
+    if ($namespace_suffix) {
+      if ($namespace_suffix && '\\' !== $namespace_suffix[0]) {
+        $namespace_suffix = '\\' . $namespace_suffix;
+      }
+      $this->namespaceSuffix = $namespace_suffix;
+      $this->directorySuffix = str_replace('\\', '/', $namespace_suffix);
+    }
+    $this->addAnnotationNamespace('Drupal\Component\Annotation');
+    $this->addAnnotationNamespace('Drupal\Core\Annotation');
+    parent::__construct($plugin_definition_annotation_name);
+  }
+
+  /**
+   * @param string $namespace
+   *   The namespace.
+   * @param bool|string|array $dir
+   *   The PSR-4 directory, or an array of PSR-4 directories, or
+   *   TRUE, to use the regular class loader via class_exists().
+   */
+  public function addAnnotationNamespace($namespace, $dir = TRUE) {
+    $this->annotationNamespaces[$namespace] = $dir;
   }
 
   /**
@@ -99,14 +126,28 @@ protected function getProviderFromNamespace($namespace) {
    */
   protected function getPluginNamespaces() {
     $plugin_namespaces = array();
-    foreach ($this->rootNamespacesIterator as $namespace => $dir) {
-      if ($this->subdir) {
-        $namespace .= "\\{$this->subdir}";
+    if ($this->namespaceSuffix) {
+      foreach ($this->rootNamespacesIterator as $namespace => $dirs) {
+        $namespace .= $this->namespaceSuffix;
+        foreach ((array) $dirs as $dir) {
+          $plugin_namespaces[$namespace][] = $dir . $this->directorySuffix;
+        }
+      }
+    }
+    else {
+      foreach ($this->rootNamespacesIterator as $namespace => $dirs) {
+        $plugin_namespaces[$namespace] = (array) $dirs;
       }
-      $plugin_namespaces[$namespace] = array($dir);
     }
 
     return $plugin_namespaces;
   }
 
+  /**
+   * @inheritdoc
+   */
+  protected function getAnnotationNamespaces() {
+    return $this->annotationNamespaces;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManager.php b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
index 6f85867..36769fd 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManager.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
@@ -49,10 +49,8 @@ public function __construct(\Traversable $namespaces, CacheBackendInterface $cac
     $this->alterInfo($module_handler, 'data_type_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'typed_data:types');
 
-    $annotation_namespaces = array(
-      'Drupal\Core\TypedData\Annotation' => DRUPAL_ROOT . '/core/lib',
-    );
-    parent::__construct('Plugin/DataType', $namespaces, $annotation_namespaces, 'Drupal\Core\TypedData\Annotation\DataType');
+    parent::__construct($namespaces, 'Plugin\DataType', 'Drupal\Core\TypedData\Annotation\DataType');
+    $this->addAnnotationNamespace('Drupal\Core\TypedData\Annotation');
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Validation/ConstraintManager.php b/core/lib/Drupal/Core/Validation/ConstraintManager.php
index 91551bc..3fe6ace 100644
--- a/core/lib/Drupal/Core/Validation/ConstraintManager.php
+++ b/core/lib/Drupal/Core/Validation/ConstraintManager.php
@@ -47,7 +47,7 @@ class ConstraintManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Plugin/Validation/Constraint', $namespaces);
+    parent::__construct($namespaces, 'Plugin\Validation\Constraint');
     $this->discovery = new StaticDiscoveryDecorator($this->discovery, array($this, 'registerDefinitions'));
     $this->alterInfo($module_handler, 'validation_constraint');
     $this->setCacheBackend($cache_backend, $language_manager, 'validation_constraint');
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php
index 6362b51..96bcddf 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php
@@ -36,11 +36,8 @@ public function __construct($type, \Traversable $namespaces, CacheBackendInterfa
       'processor' => 'Drupal\aggregator\Annotation\AggregatorProcessor',
     );
 
-    $annotation_namespaces = array(
-      'Drupal\aggregator\Annotation' => DRUPAL_ROOT . '/core/modules/aggregator/lib',
-    );
-
-    parent::__construct("Plugin/aggregator/$type", $namespaces, $annotation_namespaces, $type_annotations[$type]);
+    parent::__construct($namespaces, "Plugin\\aggregator\\$type", $type_annotations[$type]);
+    $this->addAnnotationNamespace('Drupal\aggregator\Annotation');
     $this->setCacheBackend($cache_backend, $language_manager, "aggregator_$type");
   }
 
diff --git a/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php b/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php
index ead5531..0bc661e 100644
--- a/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php
+++ b/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php
@@ -35,8 +35,8 @@ class BlockManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\block\Annotation' => $namespaces['Drupal\block']);
-    parent::__construct('Plugin/Block', $namespaces, $annotation_namespaces, 'Drupal\block\Annotation\Block');
+    parent::__construct($namespaces, 'Plugin\Block', 'Drupal\block\Annotation\Block');
+    $this->addAnnotationNamespace('Drupal\block\Annotation');
     $this->alterInfo($module_handler, 'block');
     $this->setCacheBackend($cache_backend, $language_manager, 'block_plugins');
   }
diff --git a/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php b/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php
index dc69f7e..f26238e 100644
--- a/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php
+++ b/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php
@@ -33,8 +33,8 @@ class CKEditorPluginManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\ckeditor\Annotation' => $namespaces['Drupal\ckeditor']);
-    parent::__construct('Plugin/CKEditorPlugin', $namespaces, $annotation_namespaces, 'Drupal\ckeditor\Annotation\CKEditorPlugin');
+    parent::__construct($namespaces, 'Plugin\CKEditorPlugin', 'Drupal\ckeditor\Annotation\CKEditorPlugin');
+    $this->addAnnotationNamespace('Drupal\ckeditor\Annotation');
     $this->alterInfo($module_handler, 'ckeditor_plugin_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'ckeditor_plugin');
   }
diff --git a/core/modules/comment/comment.install b/core/modules/comment/comment.install
index 04e1291..463f433 100644
--- a/core/modules/comment/comment.install
+++ b/core/modules/comment/comment.install
@@ -13,7 +13,7 @@ function comment_uninstall() {
   variable_del('comment_block_count');
   $node_types = array_keys(node_type_get_types());
   Drupal::entityManager()->addNamespaces(new ArrayIterator(array(
-    'Drupal\comment' => DRUPAL_ROOT . '/core/modules/comment/lib',
+    'Drupal\comment' => DRUPAL_ROOT . '/core/modules/comment/lib/Drupal/comment',
   )));
   drupal_classloader_register('comment', 'core/modules/comment');
   foreach ($node_types as $node_type) {
diff --git a/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php b/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php
index 79f8013..0e50ac7 100644
--- a/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php
+++ b/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php
@@ -29,8 +29,8 @@ class InPlaceEditorManager extends PluginManagerBase {
    *   keyed by the corresponding namespace to look for plugin implementations,
    */
   public function __construct(\Traversable $namespaces) {
-    $annotation_namespaces = array('Drupal\edit\Annotation' => $namespaces['Drupal\edit']);
-    $this->discovery = new AnnotatedClassDiscovery('Plugin/InPlaceEditor', $namespaces, $annotation_namespaces, 'Drupal\edit\Annotation\InPlaceEditor');
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\InPlaceEditor', 'Drupal\edit\Annotation\InPlaceEditor');
+    $this->discovery->addAnnotationNamespace('Drupal\edit\Annotation');
     $this->discovery = new AlterDecorator($this->discovery, 'edit_editor');
     $this->discovery = new CacheDecorator($this->discovery, 'edit:editor');
     $this->factory = new DefaultFactory($this->discovery);
diff --git a/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php b/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php
index 9adf267..61aca81 100644
--- a/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php
+++ b/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php
@@ -31,8 +31,8 @@ class EditorManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\editor\Annotation' => $namespaces['Drupal\editor']);
-    parent::__construct('Plugin/Editor', $namespaces, $annotation_namespaces, 'Drupal\editor\Annotation\Editor');
+    parent::__construct($namespaces, 'Plugin\Editor', 'Drupal\editor\Annotation\Editor');
+    $this->addAnnotationNamespace('Drupal\editor\Annotation');
     $this->alterInfo($module_handler, 'editor_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'editor');
   }
diff --git a/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php b/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php
index 11582f2..70e90f0 100644
--- a/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php
+++ b/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php
@@ -27,8 +27,8 @@ class SelectionPluginManager extends DefaultPluginManager {
    * {@inheritdoc}
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\entity_reference\Annotation' => $namespaces['Drupal\entity_reference']);
-    $this->discovery = new AnnotatedClassDiscovery('Plugin/entity_reference/selection', $namespaces, $annotation_namespaces, 'Drupal\entity_reference\Annotation\EntityReferenceSelection');
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\entity_reference\selection', 'Drupal\entity_reference\Annotation\EntityReferenceSelection');
+    $this->discovery->addAnnotationNamespace('Drupal\entity_reference\Annotation');
 
     // We're not using the parent constructor because we use a different factory
     // method and don't need the derivative discovery decorator.
diff --git a/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php b/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php
index c916a9f..f8b2df1 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php
@@ -54,9 +54,9 @@ class FormatterPluginManager extends DefaultPluginManager {
    *   The 'field type' plugin manager.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, LanguageManager $language_manager, FieldTypePluginManager $field_type_manager) {
-    $annotation_namespaces = array('Drupal\field\Annotation' => $namespaces['Drupal\field']);
 
-    parent::__construct('Plugin/field/formatter', $namespaces, $annotation_namespaces, 'Drupal\field\Annotation\FieldFormatter');
+    parent::__construct($namespaces, 'Plugin\field\formatter', 'Drupal\field\Annotation\FieldFormatter');
+    $this->addAnnotationNamespace('Drupal\field\Annotation');
 
     $this->setCacheBackend($cache_backend, $language_manager, 'field_formatter_types');
     $this->alterInfo($module_handler, 'field_formatter_info');
diff --git a/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php b/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php
index 0fe4be9..e7dac34 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php
@@ -54,9 +54,9 @@ class WidgetPluginManager extends DefaultPluginManager {
    *   The 'field type' plugin manager.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, LanguageManager $language_manager, FieldTypePluginManager $field_type_manager) {
-    $annotation_namespaces = array('Drupal\field\Annotation' => $namespaces['Drupal\field']);
 
-    parent::__construct('Plugin/field/widget', $namespaces, $annotation_namespaces, 'Drupal\field\Annotation\FieldWidget');
+    parent::__construct($namespaces, 'Plugin\field\widget', 'Drupal\field\Annotation\FieldWidget');
+    $this->addAnnotationNamespace('Drupal\field\Annotation');
 
     $this->setCacheBackend($cache_backend, $language_manager, 'field_widget_types');
     $this->alterInfo($module_handler, 'field_widget_info');
diff --git a/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php b/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php
index e2aae54..6892c75 100644
--- a/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php
+++ b/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php
@@ -39,8 +39,8 @@ class FilterPluginManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\filter\Annotation' => $namespaces['Drupal\filter']);
-    parent::__construct('Plugin/Filter', $namespaces, $annotation_namespaces, 'Drupal\filter\Annotation\Filter');
+    parent::__construct($namespaces, 'Plugin\Filter', 'Drupal\filter\Annotation\Filter');
+    $this->addAnnotationNamespace('Drupal\filter\Annotation');
     $this->alterInfo($module_handler, 'filter_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'filter_plugins', array('filter_formats' => TRUE));
   }
diff --git a/core/modules/image/lib/Drupal/image/ImageEffectManager.php b/core/modules/image/lib/Drupal/image/ImageEffectManager.php
index 64e251e..1bcc028 100644
--- a/core/modules/image/lib/Drupal/image/ImageEffectManager.php
+++ b/core/modules/image/lib/Drupal/image/ImageEffectManager.php
@@ -21,8 +21,8 @@ class ImageEffectManager extends DefaultPluginManager {
    * {@inheritdoc}
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\image\Annotation' => $namespaces['Drupal\image']);
-    parent::__construct('Plugin/ImageEffect', $namespaces, $annotation_namespaces, 'Drupal\image\Annotation\ImageEffect');
+    parent::__construct($namespaces, 'Plugin\ImageEffect', 'Drupal\image\Annotation\ImageEffect');
+    $this->addAnnotationNamespace('Drupal\image\Annotation');
 
     $this->alterInfo($module_handler, 'image_effect_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'image_effect');
diff --git a/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php b/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php
new file mode 100644
index 0000000..f169d06
--- /dev/null
+++ b/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php
@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\layout\Plugin\Type\LayoutManager.
+ */
+
+namespace Drupal\layout\Plugin\Type;
+
+use Drupal\Component\Plugin\PluginManagerBase;
+use Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator;
+use Drupal\Component\Plugin\Discovery\ProcessDecorator;
+use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
+use Drupal\Component\Plugin\Factory\ReflectionFactory;
+
+/**
+ * Layout plugin manager.
+ */
+class LayoutManager extends PluginManagerBase {
+
+  protected $defaults = array(
+    'class' => 'Drupal\layout\Plugin\Layout\StaticLayout',
+  );
+
+  /**
+   * Overrides Drupal\Component\Plugin\PluginManagerBase::__construct().
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations,
+   */
+  public function __construct(\Traversable $namespaces) {
+    // Create layout plugin derivatives from declaratively defined layouts.
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\Layout');
+    $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
+    $this->discovery = new ProcessDecorator($this->discovery, array($this, 'processDefinition'));
+
+    $this->factory = new ReflectionFactory($this->discovery);
+  }
+}
diff --git a/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php b/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php
index 0ad6b0f..948cad5 100644
--- a/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php
+++ b/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php
@@ -32,7 +32,7 @@ class ResourcePluginManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Plugin/rest/resource', $namespaces);
+    parent::__construct($namespaces, 'Plugin\rest\resource');
 
     $this->setCacheBackend($cache_backend, $language_manager, 'rest_plugins');
     $this->alterInfo($module_handler, 'rest_resource');
diff --git a/core/modules/search/lib/Drupal/search/SearchPluginManager.php b/core/modules/search/lib/Drupal/search/SearchPluginManager.php
index 3821791..b75f934 100644
--- a/core/modules/search/lib/Drupal/search/SearchPluginManager.php
+++ b/core/modules/search/lib/Drupal/search/SearchPluginManager.php
@@ -28,8 +28,8 @@ class SearchPluginManager extends DefaultPluginManager {
    * {@inheritdoc}
    */
   public function __construct(\Traversable $namespaces, ConfigFactory $config_factory) {
-    $annotation_namespaces = array('Drupal\search\Annotation' => $namespaces['Drupal\search']);
-    parent::__construct('Plugin/Search', $namespaces, $annotation_namespaces, 'Drupal\search\Annotation\SearchPlugin');
+    parent::__construct($namespaces, 'Plugin/Search', 'Drupal\search\Annotation\SearchPlugin');
+    $this->addAnnotationNamespace('Drupal\search\Annotation');
 
     $this->configFactory = $config_factory;
   }
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 8d441f4..5ab35db 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -454,21 +454,29 @@ function simpletest_test_get_all() {
       $all_data = $module_data + system_rebuild_theme_data();
       $all_data += drupal_system_listing('/\.profile$/', 'profiles', 'name');
       foreach ($all_data as $name => $data) {
-        // Build directory in which the test files would reside.
-        $tests_dir = DRUPAL_ROOT . '/' . dirname($data->uri) . '/lib/Drupal/' . $name . '/Tests';
+        $extension_dir = DRUPAL_ROOT . '/' . dirname($data->uri);
+
+        // Build directories in which the test files would reside.
+        $tests_dirs = array();
+        $tests_dirs[] = $extension_dir . '/lib/Drupal/' . $name . '/Tests';
+        $tests_dirs[] = $extension_dir . '/lib/Tests';
+        $tests_dirs[] = $extension_dir . '/src/Tests';
+        $tests_dirs[] = $extension_dir . '/tests/Drupal/' . $name . '/Tests';
+        $tests_dirs[] = $extension_dir . '/tests/lib';
+        $tests_dirs[] = $extension_dir . '/tests/src';
+
+        $namespace = 'Drupal\\' . $name . '\Tests\\';
+
         // Scan it for test files if it exists.
-        if (is_dir($tests_dir)) {
-          $files = file_scan_directory($tests_dir, '/.*\.php/');
-          if (!empty($files)) {
-            $basedir = DRUPAL_ROOT . '/' . dirname($data->uri) . '/lib/';
-            foreach ($files as $file) {
-              // Convert the file name into the namespaced class name.
-              $replacements = array(
-                '/' => '\\',
-                $basedir => '',
-                '.php' => '',
-              );
-              $classes[] = strtr($file->uri, $replacements);
+        foreach ($tests_dirs as $tests_dir) {
+          if (is_dir($tests_dir)) {
+            $files = file_scan_directory($tests_dir, '/.*\.php/');
+            if (!empty($files)) {
+              $strlen = strlen($tests_dir) + 1;
+              // Convert the file names into the namespaced class names.
+              foreach ($files as $file) {
+                $classes[] = $namespace . str_replace('/', '\\', substr($file->uri, $strlen, -4));
+              }
             }
           }
         }
@@ -526,8 +534,15 @@ function simpletest_classloader_register() {
   foreach ($types as $type => $info) {
     $matches = drupal_system_listing('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.' . $info['extension'] . '$/', $info['dir']);
     foreach ($matches as $name => $file) {
+      // @todo Register PSR-4 namespace directories, once our classloader supports it.
+      // See https://drupal.org/node/2058867
       drupal_classloader_register($name, dirname($file->uri));
-      $classloader->add('Drupal\\' . $name . '\\Tests', DRUPAL_ROOT . '/' . dirname($file->uri) . '/tests');
+      $extension_dir = dirname($file->uri);
+      $dirs = array();
+      $dirs[] = $extension_dir . '/tests/Drupal/' . $name . '/Tests';
+      $dirs[] = $extension_dir . '/tests/lib';
+      $dirs[] = $extension_dir . '/tests/src';
+      $classloader->addPsr4('Drupal\\' . $name . '\\Tests\\', $dirs);
       // While being there, prime drupal_get_filename().
       drupal_get_filename($type, $name, $file->uri);
     }
diff --git a/core/modules/statistics/statistics.php b/core/modules/statistics/statistics.php
index ed132e8..22acde2 100644
--- a/core/modules/statistics/statistics.php
+++ b/core/modules/statistics/statistics.php
@@ -9,7 +9,7 @@
 chdir('../../..');
 
 // Load the Drupal bootstrap.
-require_once dirname(dirname(__DIR__)) . '/vendor/autoload.php';
+require_once dirname(dirname(__DIR__)) . '/autoload.php';
 require_once dirname(dirname(__DIR__)) . '/includes/bootstrap.inc';
 drupal_bootstrap(DRUPAL_BOOTSTRAP_VARIABLES);
 
diff --git a/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php
index dc22c51..6188bef 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php
@@ -28,8 +28,8 @@ class ImageToolkitManager extends DefaultPluginManager {
    *   The language manager.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager) {
-    $annotation_namespaces = array('Drupal\system\Annotation' => $namespaces['Drupal\system']);
-    parent::__construct('Plugin/ImageToolkit', $namespaces, $annotation_namespaces, 'Drupal\system\Annotation\ImageToolkit');
+    parent::__construct($namespaces, 'Plugin\ImageToolkit', 'Drupal\system\Annotation\ImageToolkit');
+    $this->addAnnotationNamespace('Drupal\system\Annotation');
     $this->setCacheBackend($cache_backend, $language_manager, 'image_toolkit');
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php b/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php
new file mode 100644
index 0000000..971d47d
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php
@@ -0,0 +1,51 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Plugin\Type\PluginUIManager.
+ */
+
+namespace Drupal\system\Plugin\Type;
+
+use Drupal\Component\Plugin\PluginManagerBase;
+use Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator;
+use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
+use Drupal\Core\Plugin\Discovery\AlterDecorator;
+use Drupal\Core\Plugin\Discovery\CacheDecorator;
+use Drupal\Component\Plugin\Factory\DefaultFactory;
+
+/**
+ * Manages discovery and instantiation of Plugin UI plugins.
+ *
+ * @todo This class needs @see references and/or more documentation.
+ */
+class PluginUIManager extends PluginManagerBase {
+
+  /**
+   * Constructs a \Drupal\system\Plugin\Type\PluginUIManager object.
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations,
+   */
+  public function __construct(\Traversable $namespaces) {
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\PluginUI');
+    $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
+    $this->discovery = new AlterDecorator($this->discovery, 'plugin_ui');
+    $this->discovery = new CacheDecorator($this->discovery, 'plugin_ui');
+    $this->factory = new DefaultFactory($this->discovery);
+  }
+
+  /**
+   * Overrides \Drupal\Component\Plugin\PluginManagerBase::processDefinition().
+   */
+  public function processDefinition(&$definition, $plugin_id) {
+    $definition += array(
+      'default_task' => TRUE,
+      'task_title' => t('View'),
+      'task_suffix' => 'view',
+      'access_callback' => 'user_access',
+    );
+  }
+
+}
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
index d2fba9b..abb8125 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
@@ -57,9 +57,11 @@ public function setUp() {
         'provider' => 'plugin_test',
       ),
     );
-    $namespaces = new \ArrayObject(array('Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib'));
-    $this->discovery = new AnnotatedClassDiscovery('Plugin/plugin_test/fruit', $namespaces);
-    $this->emptyDiscovery = new AnnotatedClassDiscovery('Plugin/non_existing_module/non_existing_plugin_type', $namespaces);
+    $namespaces = new \ArrayObject(array(
+      'Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test',
+    ));
+    $this->discovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\plugin_test\fruit');
+    $this->emptyDiscovery = new AnnotatedClassDiscovery($namespaces, 'Plugin\non_existing_module\non_existing_plugin_type');
   }
 
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php
index b572919..674862b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php
@@ -41,13 +41,15 @@ protected function setUp() {
         'provider' => 'plugin_test',
       ),
     );
-    $root_namespaces = new \ArrayObject(array('Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib'));
-    $annotation_namespaces = array(
-      'Drupal\plugin_test\Plugin\Annotation' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib',
-    );
+    $root_namespaces = new \ArrayObject(array(
+      'Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test'
+    ));
+
+    $this->discovery = new AnnotatedClassDiscovery($root_namespaces, 'Plugin\plugin_test\custom_annotation', 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
+    $this->discovery->addAnnotationNamespace('Drupal\plugin_test\Plugin\Annotation');
 
-    $this->discovery = new AnnotatedClassDiscovery('Plugin/plugin_test/custom_annotation', $root_namespaces, $annotation_namespaces, 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
-    $this->emptyDiscovery = new AnnotatedClassDiscovery('Plugin/non_existing_module/non_existing_plugin_type', $root_namespaces, $annotation_namespaces, 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
+    $this->emptyDiscovery = new AnnotatedClassDiscovery($root_namespaces, 'Plugin\non_existing_module\non_existing_plugin_type', 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
+    $this->emptyDiscovery->addAnnotationNamespace('Drupal\plugin_test\Plugin\Annotation');
   }
 
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php
index c005300..f0d3613 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php
@@ -39,10 +39,13 @@ protected function setUp() {
         'provider' => 'plugin_test',
       ),
     );
-    $namespaces = new \ArrayObject(array('Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib'));
-    $this->discovery = new AnnotatedClassDiscovery('', $namespaces);
+
+    $namespaces = new \ArrayObject(array(
+      'Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test',
+    ));
+    $this->discovery = new AnnotatedClassDiscovery($namespaces);
     $empty_namespaces = new \ArrayObject();
-    $this->emptyDiscovery = new AnnotatedClassDiscovery('', $empty_namespaces);
+    $this->emptyDiscovery = new AnnotatedClassDiscovery($empty_namespaces);
   }
 
 }
diff --git a/core/modules/system/tests/http.php b/core/modules/system/tests/http.php
index 662031f..10f4e20 100644
--- a/core/modules/system/tests/http.php
+++ b/core/modules/system/tests/http.php
@@ -18,6 +18,6 @@
 
 // Change current directory to the Drupal root.
 chdir('../../../..');
-require_once dirname(dirname(dirname(__DIR__))) . '/vendor/autoload.php';
+require_once dirname(dirname(dirname(__DIR__))) . '/autoload.php';
 require_once dirname(dirname(dirname(__DIR__))) . '/includes/bootstrap.inc';
 drupal_handle_request(TRUE);
diff --git a/core/modules/system/tests/https.php b/core/modules/system/tests/https.php
index 247e6e5..54c8afa 100644
--- a/core/modules/system/tests/https.php
+++ b/core/modules/system/tests/https.php
@@ -20,6 +20,6 @@
 
 // Change current directory to the Drupal root.
 chdir('../../../..');
-require_once dirname(dirname(dirname(__DIR__))) . '/vendor/autoload.php';
+require_once dirname(dirname(dirname(__DIR__))) . '/autoload.php';
 require_once dirname(dirname(dirname(__DIR__))) . '/includes/bootstrap.inc';
 drupal_handle_request(TRUE);
diff --git a/core/modules/taxonomy/taxonomy.install b/core/modules/taxonomy/taxonomy.install
index 7648ba0..2752cf7 100644
--- a/core/modules/taxonomy/taxonomy.install
+++ b/core/modules/taxonomy/taxonomy.install
@@ -14,7 +14,7 @@
  */
 function taxonomy_uninstall() {
   Drupal::entityManager()->addNamespaces(new ArrayIterator(array(
-    'Drupal\taxonomy' => DRUPAL_ROOT . '/core/modules/taxonomy/lib',
+    'Drupal\taxonomy' => DRUPAL_ROOT . '/core/modules/taxonomy/lib/Drupal/taxonomy',
   )));
   drupal_classloader_register('taxonomy', 'core/modules/taxonomy');
   // Remove taxonomy_term bundles.
diff --git a/core/modules/tour/lib/Drupal/tour/TipPluginManager.php b/core/modules/tour/lib/Drupal/tour/TipPluginManager.php
index fb18d3b..88e0784 100644
--- a/core/modules/tour/lib/Drupal/tour/TipPluginManager.php
+++ b/core/modules/tour/lib/Drupal/tour/TipPluginManager.php
@@ -31,8 +31,8 @@ class TipPluginManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\tour\Annotation' => $namespaces['Drupal\tour']);
-    parent::__construct('Plugin/tour/tip', $namespaces, $annotation_namespaces, 'Drupal\tour\Annotation\Tip');
+    parent::__construct($namespaces, 'Plugin\tour\tip', 'Drupal\tour\Annotation\Tip');
+    $this->addAnnotationNamespace('Drupal\tour\Annotation');
 
     $this->alterInfo($module_handler, 'tour_tips_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'tour');
diff --git a/core/modules/views/lib/Drupal/views/Plugin/Discovery/ViewsHandlerDiscovery.php b/core/modules/views/lib/Drupal/views/Plugin/Discovery/ViewsHandlerDiscovery.php
index 460ef5a..c9a15ef 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/Discovery/ViewsHandlerDiscovery.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/Discovery/ViewsHandlerDiscovery.php
@@ -22,13 +22,6 @@ class ViewsHandlerDiscovery extends AnnotatedClassDiscovery {
   protected $type;
 
   /**
-   * An object containing the namespaces to look for plugin implementations.
-   *
-   * @var \Traversable
-   */
-  protected $rootNamespacesIterator;
-
-  /**
    * Constructs a ViewsHandlerDiscovery object.
    *
    * @param string $type
@@ -39,19 +32,9 @@ class ViewsHandlerDiscovery extends AnnotatedClassDiscovery {
    */
   function __construct($type, \Traversable $root_namespaces) {
     $this->type = $type;
-    $this->rootNamespacesIterator = $root_namespaces;
 
-    $annotation_namespaces = array(
-      'Drupal\Component\Annotation' => DRUPAL_ROOT . '/core/lib',
-    );
-    $plugin_namespaces = array();
-    foreach ($root_namespaces as $namespace => $dir) {
-      $plugin_namespaces["$namespace\\Plugin\\views\\{$type}"] = array($dir);
-    }
-
-    $this->pluginNamespaces = $plugin_namespaces;
-    $this->annotationNamespaces = $annotation_namespaces;
-    $this->pluginDefinitionAnnotationName = 'Drupal\Component\Annotation\PluginID';
+    parent::__construct($root_namespaces, 'Plugin\views\\' . $type, 'Drupal\Component\Annotation\PluginID');
+    unset($this->annotationNamespaces['Drupal\Core\Annotation']);
   }
 
   /**
@@ -66,16 +49,4 @@ public function getDefinitions() {
     return $definitions;
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function getPluginNamespaces() {
-    $plugin_namespaces = array();
-    foreach ($this->rootNamespacesIterator as $namespace => $dir) {
-      $plugin_namespaces["$namespace\\Plugin\\views\\{$this->type}"] = array($dir);
-    }
-
-    return $plugin_namespaces;
-  }
-
 }
diff --git a/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php b/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php
index d603fba..4404bf5 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php
@@ -34,9 +34,9 @@ class ViewsPluginManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct($type, \Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $annotation_namespaces = array('Drupal\views\Annotation' => $namespaces['Drupal\views']);
     $plugin_definition_annotation_name = 'Drupal\views\Annotation\Views' . Container::camelize($type);
-    parent::__construct("Plugin/views/$type", $namespaces, $annotation_namespaces, $plugin_definition_annotation_name);
+    parent::__construct($namespaces, "Plugin\\views\\$type", $plugin_definition_annotation_name);
+    $this->addAnnotationNamespace('Drupal\views\Annotation');
 
     $this->defaults += array(
       'parent' => 'parent',
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 31ea371..d274252 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -4,7 +4,7 @@
  * This script runs Drupal tests from command line.
  */
 
-require_once __DIR__ . '/../vendor/autoload.php';
+require_once __DIR__ . '/../autoload.php';
 
 use Drupal\Core\StreamWrapper\PublicStream;
 
diff --git a/core/scripts/switch-psr4.sh b/core/scripts/switch-psr4.sh
new file mode 100644
index 0000000..3cec824
--- /dev/null
+++ b/core/scripts/switch-psr4.sh
@@ -0,0 +1,108 @@
+#!/bin/php
+<?php
+
+/**
+ * @file
+ * Moves class files in extensions to their PSR-4 location.
+ */
+
+// Determine DRUPAL_ROOT.
+$dir = dirname(__FILE__);
+while (!defined('DRUPAL_ROOT')) {
+  if (is_dir($dir . '/core')) {
+    define('DRUPAL_ROOT', $dir);
+  }
+  $dir = dirname($dir);
+}
+
+process_extensions_base_dir(DRUPAL_ROOT . '/core/modules');
+process_extensions_base_dir(DRUPAL_ROOT . '/core/profiles');
+
+function process_extensions_base_dir($dir) {
+  /**
+   * @var SplFileInfo $fileinfo
+   */
+  foreach (new \DirectoryIterator($dir) as $fileinfo) {
+    if ($fileinfo->isDot()) {
+      // do nothing
+    }
+    elseif ($fileinfo->isDir()) {
+      process_candidate_dir($fileinfo->getPathname());
+    }
+  }
+}
+
+function process_candidate_dir($dir) {
+  foreach (new \DirectoryIterator($dir) as $fileinfo) {
+    if ($fileinfo->isDot()) {
+      // Ignore "." and "..".
+    }
+    elseif ($fileinfo->isDir()) {
+      // It's a directory.
+      switch ($fileinfo->getFilename()) {
+        case 'lib':
+        case 'src':
+        case 'module_autoload_test':
+          // Ignore these directory names.
+          continue;
+        default:
+          // Look for more extensions in subdirectories.
+          process_candidate_dir($fileinfo->getPathname());
+      }
+    }
+    else {
+      // It's a file.
+      if (preg_match('/^(.+).info.yml$/', $fileinfo->getFilename(), $m)) {
+        // It's a *.info.yml file, so we found an extension directory.
+        $extension_name = $m[1];
+      }
+    }
+  }
+  if (isset($extension_name)) {
+    process_extension($extension_name, $dir);
+  }
+}
+
+function process_extension($name, $dir) {
+
+  // Move main module class files.
+  if (is_dir("$dir/lib/Drupal/$name")) {
+    // This is a module directory with a PSR-0 /lib/ folder.
+    if (!is_dir($dir . '/src')) {
+      mkdir($dir . '/src');
+    }
+    rename("$dir/lib/Drupal/$name", "$dir/src");
+  }
+
+  // Move class files in tests directory.
+  if (is_dir("$dir/tests/Drupal/$name/Tests")) {
+    rename("$dir/tests/Drupal/$name/Tests", "$dir/tests/src");
+  }
+
+  // Clean up empty directories.
+  foreach (array(
+    "lib/Drupal/$name",
+    'lib/Drupal',
+    'lib',
+    "tests/Drupal/$name/Tests",
+    "tests/Drupal/$name",
+    "tests/Drupal",
+  ) as $subdir) {
+    if (is_dir_empty("$dir/$subdir")) {
+      rmdir("$dir/$subdir");
+    }
+  }
+}
+
+function is_dir_empty($dir) {
+  if (!is_readable($dir)) {
+    return NULL;
+  }
+  $handle = opendir($dir);
+  while (false !== ($entry = readdir($handle))) {
+    if ($entry != "." && $entry != "..") {
+      return FALSE;
+    }
+  }
+  return TRUE;
+}
diff --git a/core/tests/bootstrap.php b/core/tests/bootstrap.php
index 66c4be4..e0be3f8 100644
--- a/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -1,7 +1,7 @@
 <?php
 
 // Register the namespaces we'll need to autoload from.
-$loader = require __DIR__ . "/../vendor/autoload.php";
+$loader = require __DIR__ . "/../autoload.php";
 $loader->add('Drupal\\Tests', __DIR__);
 
 foreach (scandir(__DIR__ . "/../modules") as $module) {
diff --git a/core/update.php b/core/update.php
index ac36b2a..0238871 100644
--- a/core/update.php
+++ b/core/update.php
@@ -22,7 +22,7 @@
 // Change the directory to the Drupal root.
 chdir('..');
 
-require_once __DIR__ . '/vendor/autoload.php';
+require_once __DIR__ . '/autoload.php';
 
 // Exit early if an incompatible PHP version would cause fatal errors.
 // The minimum version is specified explicitly, as DRUPAL_MINIMUM_PHP is not
diff --git a/index.php b/index.php
index 426aa22..5f5ada6 100644
--- a/index.php
+++ b/index.php
@@ -8,7 +8,7 @@
  * See COPYRIGHT.txt and LICENSE.txt files in the "core" directory.
  */
 
-require_once __DIR__ . '/core/vendor/autoload.php';
+require_once __DIR__ . '/core/autoload.php';
 require_once __DIR__ . '/core/includes/bootstrap.inc';
 
 try {