diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1448,7 +1448,7 @@
 function install_bootstrap_full(&$install_state) {
   // Clear the module list that was overriden earlier in the process.
   // This will allow all freshly installed modules to be loaded.
-  drupal_static_reset('module_list');
+  module_list_reset();
   drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
 }
 
diff -u b/core/includes/module.inc b/core/includes/module.inc
--- b/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -45,54 +45,64 @@
  *
  * Acts as a wrapper around system_list(), returning either a list of all
  * enabled modules, or just modules needed for bootstrap.
- * An optional parameter allows an alternative module list to be provided,
- * which is then stored and used in subsequent calls instead of the one
- * provided by system_list().
  *
- * @param $type
+ * The returned module list is always based on system_list(). The only exception
+ * to that is when a fixed list of modules has been passed in previously, in
+ * which case system_list() is omitted and the fixed list is always returned in
+ * subsequent calls until manually reverted via module_list_reset().
+ *
+ * @param string $type
  *   The type of list to return:
  *   - module_enabled: All enabled modules.
  *   - bootstrap: All enabled modules required for bootstrap.
- * @param $fixed_list
- *   (optional) If an array of module names is provided, this will override the
- *   module list with the given set of modules. This will persist until the next
- *   call with a new $fixed_list passed in. This parameter is primarily
- *   intended for internal use (e.g., in install.php and update.php).
+ * @param array $fixed_list
+ *   (optional) An array of module names to override the list of modules. This
+ *   list will persist until the next call with a new $fixed_list passed in.
+ *   Primarily intended for internal use (e.g., in install.php and update.php).
+ *   Use module_list_reset() to undo the $fixed_list override.
  *
- * @return
+ * @return array
  *   An associative array whose keys and values are the names of the modules in
  *   the list.
  */
-function module_list($type = 'module_enabled', $fixed_list = NULL) {
-  // Tracks data acquired from $fixed_list.
+function module_list($type = 'module_enabled', array $fixed_list = NULL) {
+  // This static is only used for $fixed_list.
   $module_list = &drupal_static(__FUNCTION__);
 
   // The list that will be be returned. Separate from $module_list in order
-  // to avoid statically caching data acquired from system_list(), since
-  // that function already has its own static cache.
+  // to not duplicate the static cache of system_list().
   $list = $module_list;
 
   if (isset($fixed_list)) {
     foreach ($fixed_list as $name => $module) {
       drupal_get_filename('module', $name, $module['filename']);
       $module_list[$name] = $name;
-     }
+    }
     $list = $module_list;
   }
   elseif (!isset($module_list)) {
-   if ($type == 'bootstrap') {
-     $list = system_list('bootstrap');
-   }
-   else {
-     // Not using drupal_map_assoc() here as that requires common.inc.
-     $list = array_keys(system_list($type));
-     $list = (!empty($list) ? array_combine($list, $list) : array());
+    if ($type == 'bootstrap') {
+      $list = system_list('bootstrap');
+    }
+    else {
+      // Not using drupal_map_assoc() here as that requires common.inc.
+      $list = array_keys(system_list($type));
+      $list = (!empty($list) ? array_combine($list, $list) : array());
     }
   }
   return $list;
 }
 
 /**
+ * Reverts an enforced fixed list of module_list().
+ *
+ * Subsequent calls to module_list() will no longer use a fixed list.
+ */
+function module_list_reset() {
+  drupal_static_reset('module_list');
+}
+
+/**
  * Build a list of bootstrap modules and enabled modules and themes.
  *
  * @param $type
@@ -660,8 +670,7 @@
     $implementations['#write_cache'] = TRUE;
     $hook_info = module_hook_info();
     $implementations[$hook] = array();
-    $list = module_list();
-    foreach ($list as $module) {
+    foreach (module_list() as $module) {
       $include_file = isset($hook_info[$hook]['group']) && module_load_include('inc', $module, $module . '.' . $hook_info[$hook]['group']);
       // Since module_hook() may needlessly try to load the include file again,
       // function_exists() is used directly here.
reverted:
--- b/core/modules/search/lib/Drupal/search/Tests/SearchExcerptTest.php
+++ a/core/modules/search/lib/Drupal/search/Tests/SearchExcerptTest.php
@@ -7,12 +7,12 @@
 
 namespace Drupal\search\Tests;
 
+use Drupal\simpletest\UnitTestBase;
-use Drupal\simpletest\WebTestBase;
 
 /**
  * Tests the search_excerpt() function.
  */
+class SearchExcerptTest extends UnitTestBase {
-class SearchExcerptTest extends WebTestBase {
   public static function getInfo() {
     return array(
       'name' => 'Search excerpt extraction',
@@ -21,6 +21,11 @@
     );
   }
 
+  function setUp() {
+    drupal_load('module', 'search');
+    parent::setUp();
+  }
+
   /**
    * Tests search_excerpt() with several simulated search keywords.
    *
diff -u b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -776,7 +776,8 @@
 
     // Reload module list and implementations to ensure that test module hooks
     // aren't called after tests.
-    drupal_static_reset('module_list');
+    drupal_static_reset('system_list');
+    module_list_reset();
     module_implements_reset();
 
     // Reset the Field API.
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Module/ClassLoaderTest.php
+++ /dev/null
@@ -1,48 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\system\Tests\Module\ClassLoaderTest.
- */
-
-namespace Drupal\system\Tests\Module;
-
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Tests class loading.
- */
-class ClassLoaderTest extends WebTestBase {
-  protected $profile = 'testing';
-
-  public static function getInfo() {
-    return array(
-      'name' => 'Module class loader',
-      'description' => 'Tests class loading for modules.',
-      'group' => 'Module',
-    );
-  }
-
-  /**
-   * Tests that module-provided classes can be loaded when a module is enabled.
-   */
-  function testClassLoading() {
-    $expected = 'Drupal\\module_autoload_test\\SomeClass::testMethod() was invoked.';
-
-    module_enable(array('module_test', 'module_autoload_test'), FALSE);
-    $this->resetAll();
-    // Check twice to test an unprimed and primed system_list() cache.
-    for ($i=0; $i<2; $i++) {
-      $this->drupalGet('module-test/class-loading');
-      $this->assertText($expected, t('Autoloader loads classes from an enabled module.'));
-    }
-
-    module_disable(array('module_autoload_test'), FALSE);
-    $this->resetAll();
-    // Check twice to test an unprimed and primed system_list() cache.
-    for ($i=0; $i<2; $i++) {
-      $this->drupalGet('module-test/class-loading');
-      $this->assertNoText($expected, t('Autoloader does not load classes from a disabled module.'));
-    }
-  }
-}
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Module/InstallTest.php
+++ /dev/null
@@ -1,47 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\system\Tests\Module\InstallTest.
- */
-
-namespace Drupal\system\Tests\Module;
-
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Unit tests for module installation.
- */
-class InstallTest extends WebTestBase {
-  public static function getInfo() {
-    return array(
-      'name' => 'Module installation',
-      'description' => 'Tests the installation of modules.',
-      'group' => 'Module',
-    );
-  }
-
-  function setUp() {
-    parent::setUp('module_test');
-  }
-
-  /**
-   * Test that calls to drupal_write_record() work during module installation.
-   *
-   * This is a useful function to test because modules often use it to insert
-   * initial data in their database tables when they are being installed or
-   * enabled. Furthermore, drupal_write_record() relies on the module schema
-   * information being available, so this also checks that the data from one of
-   * the module's hook implementations, in particular hook_schema(), is
-   * properly available during this time. Therefore, this test helps ensure
-   * that modules are fully functional while Drupal is installing and enabling
-   * them.
-   */
-  function testDrupalWriteRecord() {
-    // Check for data that was inserted using drupal_write_record() while the
-    // 'module_test' module was being installed and enabled.
-    $data = db_query("SELECT data FROM {module_test}")->fetchCol();
-    $this->assertTrue(in_array('Data inserted in hook_install()', $data), t('Data inserted using drupal_write_record() in hook_install() is correctly saved.'));
-    $this->assertTrue(in_array('Data inserted in hook_enable()', $data), t('Data inserted using drupal_write_record() in hook_enable() is correctly saved.'));
-  }
-}
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Module/UninstallTest.php
+++ /dev/null
@@ -1,41 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\system\Tests\Module\UninstallTest.
- */
-
-namespace Drupal\system\Tests\Module;
-
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Unit tests for module uninstallation and related hooks.
- */
-class UninstallTest extends WebTestBase {
-  public static function getInfo() {
-    return array(
-      'name' => 'Module uninstallation',
-      'description' => 'Tests the uninstallation of modules.',
-      'group' => 'Module',
-    );
-  }
-
-  function setUp() {
-    parent::setUp('module_test', 'user');
-  }
-
-  /**
-   * Tests the hook_modules_uninstalled() of the user module.
-   */
-  function testUserPermsUninstalled() {
-    // Uninstalls the module_test module, so hook_modules_uninstalled()
-    // is executed.
-    module_disable(array('module_test'));
-    drupal_uninstall_modules(array('module_test'));
-
-    // Are the perms defined by module_test removed from {role_permission}.
-    $count = db_query("SELECT COUNT(rid) FROM {role_permission} WHERE permission = :perm", array(':perm' => 'module_test perm'))->fetchField();
-    $this->assertEqual(0, $count, t('Permissions were all removed.'));
-  }
-}
diff -u b/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
--- b/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -1198,7 +1198,6 @@
 
   // Gets module list after install process, flushes caches and displays a
   // message if there are changes.
-  drupal_static_reset('system_list');
   $post_install_list = module_list();
   if ($pre_install_list != $post_install_list) {
     drupal_flush_all_caches();
diff -u b/core/modules/system/system.install b/core/modules/system/system.install
--- b/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -514,6 +514,7 @@
   // Clear out module list and hook implementation statics before calling
   // system_rebuild_theme_data().
   drupal_static_reset('system_list');
+  module_list_reset();
   module_implements_reset();
 
   // Load system theme data appropriately.
reverted:
--- b/core/modules/system/tests/module.test
+++ a/core/modules/system/tests/module.test
@@ -53,7 +53,7 @@
       ->condition('type', 'module')
       ->execute();
     // Reset the module list.
+    module_list(TRUE);
-    system_list_reset();
     // Move contact to the end of the array.
     unset($module_list[array_search('contact', $module_list)]);
     $module_list[] = 'contact';
@@ -64,12 +64,12 @@
       'system' => array('filename' => drupal_get_path('module', 'system')),
       'menu' => array('filename' => drupal_get_path('module', 'menu')),
     );
+    module_list(FALSE, FALSE, FALSE, $fixed_list);
-    module_list(NULL, $fixed_list);
     $new_module_list = array_combine(array_keys($fixed_list), array_keys($fixed_list));
     $this->assertModuleList($new_module_list, t('When using a fixed list'));
 
     // Reset the module list.
+    module_list(TRUE);
-    drupal_static_reset('module_list');
     $this->assertModuleList($module_list, t('After reset'));
   }
 
@@ -82,6 +82,8 @@
   protected function assertModuleList(Array $expected_values, $condition) {
     $expected_values = array_combine($expected_values, $expected_values);
     $this->assertEqual($expected_values, module_list(), t('@condition: module_list() returns correct results', array('@condition' => $condition)));
+    ksort($expected_values);
+    $this->assertIdentical($expected_values, module_list(FALSE, FALSE, TRUE), t('@condition: module_list() returns correctly sorted results', array('@condition' => $condition)));
   }
 
   /**
only in patch2:
unchanged:
--- a/core/modules/system/lib/Drupal/system/Tests/Module/ClassLoaderTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Module/ClassLoaderTest.php
@@ -13,8 +13,6 @@ use Drupal\simpletest\WebTestBase;
  * Tests class loading.
  */
 class ClassLoaderTest extends WebTestBase {
-  protected $profile = 'testing';
-
   public static function getInfo() {
     return array(
       'name' => 'Module class loader',
only in patch2:
unchanged:
--- a/core/modules/system/lib/Drupal/system/Tests/Module/ModuleApiTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Module/ModuleApiTest.php
@@ -55,7 +55,7 @@ class ModuleApiTest extends WebTestBase {
       ->condition('type', 'module')
       ->execute();
     // Reset the module list.
-    module_list(TRUE);
+    system_list_reset();
     // Move contact to the end of the array.
     unset($module_list[array_search('contact', $module_list)]);
     $module_list[] = 'contact';
@@ -66,12 +66,12 @@ class ModuleApiTest extends WebTestBase {
       'system' => array('filename' => drupal_get_path('module', 'system')),
       'menu' => array('filename' => drupal_get_path('module', 'menu')),
     );
-    module_list(FALSE, FALSE, FALSE, $fixed_list);
+    module_list(NULL, $fixed_list);
     $new_module_list = array_combine(array_keys($fixed_list), array_keys($fixed_list));
     $this->assertModuleList($new_module_list, t('When using a fixed list'));
 
     // Reset the module list.
-    module_list(TRUE);
+    module_list_reset();
     $this->assertModuleList($module_list, t('After reset'));
   }
 
@@ -84,8 +84,6 @@ class ModuleApiTest extends WebTestBase {
   protected function assertModuleList(Array $expected_values, $condition) {
     $expected_values = array_combine($expected_values, $expected_values);
     $this->assertEqual($expected_values, module_list(), t('@condition: module_list() returns correct results', array('@condition' => $condition)));
-    ksort($expected_values);
-    $this->assertIdentical($expected_values, module_list(FALSE, FALSE, TRUE), t('@condition: module_list() returns correctly sorted results', array('@condition' => $condition)));
   }
 
   /**