diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index dd8e9be..1399862 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -580,14 +580,14 @@ function theme($hook, $variables = array()) {
     throw new Exception(t('theme() may not be called until all modules are loaded.'));
   }
 
-  /** @var \Drupal\Core\Utility\ThemeRegistry $hooks */
-  $hooks = \Drupal::service('theme.registry')->getRuntime();
+  /** @var \Drupal\Core\Utility\ThemeRegistry $theme_registry */
+  $theme_registry = \Drupal::service('theme.registry')->getRuntime();
 
   // If an array of hook candidates were passed, use the first one that has an
   // implementation.
   if (is_array($hook)) {
     foreach ($hook as $candidate) {
-      if ($hooks->has($candidate)) {
+      if ($theme_registry->has($candidate)) {
         break;
       }
     }
@@ -599,16 +599,16 @@ function theme($hook, $variables = array()) {
 
   // If there's no implementation, check for more generic fallbacks. If there's
   // still no implementation, log an error and return an empty string.
-  if (!$hooks->has($hook)) {
+  if (!$theme_registry->has($hook)) {
     // Iteratively strip everything after the last '__' delimiter, until an
     // implementation is found.
     while ($pos = strrpos($hook, '__')) {
       $hook = substr($hook, 0, $pos);
-      if ($hooks->has($hook)) {
+      if ($theme_registry->has($hook)) {
         break;
       }
     }
-    if (!$hooks->has($hook)) {
+    if (!$theme_registry->has($hook)) {
       // Only log a message when not trying theme suggestions ($hook being an
       // array).
       if (!isset($candidate)) {
@@ -621,7 +621,7 @@ function theme($hook, $variables = array()) {
     }
   }
 
-  $info = $hooks->get($hook);
+  $info = $theme_registry->get($hook);
   global $theme_path;
   $temp = $theme_path;
   // point path_to_theme() to the currently used theme path:
@@ -694,8 +694,8 @@ function theme($hook, $variables = array()) {
   // 'node__article' as a suggestion via hook_theme_suggestions_HOOK_alter(),
   // enabling a theme to have an alternate template file for article nodes.
   foreach (array_reverse($suggestions) as $suggestion) {
-    if ($hooks->has($suggestion)) {
-      $info = $hooks->get($suggestion);
+    if ($theme_registry->has($suggestion)) {
+      $info = $theme_registry->get($suggestion);
       break;
     }
   }
@@ -703,7 +703,7 @@ function theme($hook, $variables = array()) {
   // Invoke the variable preprocessors, if any.
   if (isset($info['base hook'])) {
     $base_hook = $info['base hook'];
-    $base_hook_info = $hooks->get($base_hook);
+    $base_hook_info = $theme_registry->get($base_hook);
     // Include files required by the base hook, since its variable preprocessors
     // might reside there.
     if (!empty($base_hook_info['includes'])) {
diff --git a/core/includes/update.inc b/core/includes/update.inc
index c616e08..7b8a6e2 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -459,7 +459,8 @@ function update_prepare_d8_bootstrap() {
   $kernel = new DrupalKernel('update', drupal_classloader(), FALSE);
   $kernel->boot();
 
-  // Remove the theme_registry cache entry from D7.
+  // Clear the D7 caches, to ensure that for example the theme_registry does not
+  // take part in the upgrade process.
   Drupal::cache('cache')->deleteAll();
 }
 
diff --git a/core/lib/Drupal/Core/Path/AliasWhitelist.php b/core/lib/Drupal/Core/Path/AliasWhitelist.php
index d43f83e..9e3a6a1 100644
--- a/core/lib/Drupal/Core/Path/AliasWhitelist.php
+++ b/core/lib/Drupal/Core/Path/AliasWhitelist.php
@@ -88,20 +88,20 @@ protected function loadMenuPathRoots() {
   /**
    * {@inheritdoc}
    */
-  public function get($key) {
+  public function get($offset) {
     $this->lazyLoadCache();
     // url() may be called with paths that are not represented by menu router
     // items such as paths that will be rewritten by hook_url_outbound_alter().
     // Therefore internally TRUE is used to indicate whitelisted paths. FALSE is
     // used to indicate paths that have already been checked but are not
     // whitelisted, and NULL indicates paths that have not been checked yet.
-    if (isset($this->storage[$key])) {
-      if ($this->storage[$key]) {
+    if (isset($this->storage[$offset])) {
+      if ($this->storage[$offset]) {
         return TRUE;
       }
     }
-    elseif (array_key_exists($key, $this->storage)) {
-      return $this->resolveCacheMiss($key);
+    elseif (array_key_exists($offset, $this->storage)) {
+      return $this->resolveCacheMiss($offset);
     }
   }
 
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 3893338..0a82082 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -210,7 +210,7 @@ public function get() {
       $this->registry = $cache->data;
     }
     else {
-      $this->registry = $this->build($this->theme, $this->baseThemes, $this->engine);
+      $this->registry = $this->build();
       // Only persist it if all modules are loaded to ensure it is complete.
       if ($this->moduleHandler->isLoaded()) {
         $this->setCache();
@@ -304,7 +304,7 @@ public function getBaseHook($hook) {
    * @return \Drupal\Core\Utility\ThemeRegistry
    *   The build theme registry.
    */
-  protected function build($theme, array $base_theme, $theme_engine) {
+  protected function build() {
     $cache = array();
     // First, preprocess the theme hooks advertised by modules. This will
     // serve as the basic registry. Since the list of enabled modules is the
@@ -324,22 +324,22 @@ protected function build($theme, array $base_theme, $theme_engine) {
     }
 
     // Process each base theme.
-    foreach ($base_theme as $base) {
+    foreach ($this->baseThemes as $base) {
       // If the base theme uses a theme engine, process its hooks.
       $base_path = dirname($base->filename);
-      if ($theme_engine) {
-        $this->processExtension($cache, $theme_engine, 'base_theme_engine', $base->name, $base_path);
+      if ($this->engine) {
+        $this->processExtension($cache, $this->engine, 'base_theme_engine', $base->name, $base_path);
       }
       $this->processExtension($cache, $base->name, 'base_theme', $base->name, $base_path);
     }
 
     // And then the same thing, but for the theme.
-    if ($theme_engine) {
-      $this->processExtension($cache, $theme_engine, 'theme_engine', $theme->name, dirname($theme->filename));
+    if ($this->engine) {
+      $this->processExtension($cache, $this->engine, 'theme_engine', $this->theme->name, dirname($this->theme->filename));
     }
 
     // Finally, hooks provided by the theme itself.
-    $this->processExtension($cache, $theme->name, 'theme', $theme->name, dirname($theme->filename));
+    $this->processExtension($cache, $this->theme->name, 'theme', $this->theme->name, dirname($this->theme->filename));
 
     // Let modules alter the registry.
     $this->moduleHandler->alter('theme_registry', $cache);
diff --git a/core/modules/locale/lib/Drupal/locale/LocaleLookup.php b/core/modules/locale/lib/Drupal/locale/LocaleLookup.php
index 54b3495..2003e4e 100644
--- a/core/modules/locale/lib/Drupal/locale/LocaleLookup.php
+++ b/core/modules/locale/lib/Drupal/locale/LocaleLookup.php
@@ -82,10 +82,10 @@ public function __construct($langcode, $context, StringStorageInterface $string_
   /**
    * {@inheritdoc}
    */
-  protected function resolveCacheMiss($key) {
+  protected function resolveCacheMiss($offset) {
     $translation = $this->stringStorage->findTranslation(array(
       'language' => $this->langcode,
-      'source' => $key,
+      'source' => $offset,
       'context' => $this->context,
     ));
 
@@ -96,19 +96,19 @@ protected function resolveCacheMiss($key) {
       // We don't have the source string, update the {locales_source} table to
       // indicate the string is not translated.
       $this->stringStorage->createString(array(
-        'source' => $key,
+        'source' => $offset,
         'context' => $this->context,
         'version' => \Drupal::VERSION
       ))->addLocation('path', request_uri())->save();
       $value = TRUE;
     }
-    $this->storage[$key] = $value;
+    $this->storage[$offset] = $value;
     // Disabling the usage of string caching allows a module to watch for
     // the exact list of strings used on a page. From a performance
     // perspective that is a really bad idea, so we have no user
     // interface for this. Be careful when turning this option off!
     if (\Drupal::config('locale.settings')->get('cache_strings')) {
-      $this->persist($key);
+      $this->persist($offset);
     }
     return $value;
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
index b6c76c8..5689b3d 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
@@ -82,9 +82,9 @@ function testTwigCacheOverride() {
       ->set('default', 'test_theme')
       ->save();
 
-    // Unset the global variables, so \Drupal\Core\Theme::init() fires
-    // drupal_theme_initialize, which fills up the global variables properly and
-    // choosed the current active theme.
+    // Unset the global variables, so \Drupal\Core\Theme\Registry::init() fires
+    // drupal_theme_initialize, which fills up the global variables  properly
+    // and chosen the current active theme.
     unset($GLOBALS['theme_info']);
     unset($GLOBALS['theme']);
     // Reset the theme registry, so that the new theme is used.
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index ad29ada..3e0104b 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1348,7 +1348,7 @@ function hook_theme($existing, $type, $theme, $path) {
  *
  * The $theme_registry array is keyed by theme hook name, and contains the
  * information returned from hook_theme(), as well as additional properties
- * added by \Drupal\Core\Theme\Registry::_theme_process_registry().
+ * added by \Drupal\Core\Theme\Registry::processExtension().
  *
  * For example:
  * @code
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index 8445150..2ec8003 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -35,7 +35,7 @@ class RegistryTest extends UnitTestCase {
   protected $cache;
 
   /**
-   * The mocked lock backend..
+   * The mocked lock backend.
    *
    * @var \Drupal\Core\Lock\LockBackendInterface|\PHPUnit_Framework_MockObject_MockObject
    */
@@ -48,6 +48,9 @@ class RegistryTest extends UnitTestCase {
    */
   protected $moduleHandler;
 
+  /**
+   * {@inheritdoc}
+   */
   public static function getInfo() {
     return array(
       'name' => 'Theme Registry',
@@ -56,6 +59,9 @@ public static function getInfo() {
     );
   }
 
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     $this->cache = $this->getMock('Drupal\Core\Cache\CacheBackendInterface');
     $this->lock = $this->getMock('Drupal\Core\Lock\LockBackendInterface');
@@ -64,6 +70,9 @@ protected function setUp() {
     $this->setupTheme();
   }
 
+  /**
+   * Tests getting the theme registry defined by a module.
+   */
   public function testGetRegistryForModule() {
     $this->setupTheme('test_theme');
     $this->registry->setTheme((object) array(