diff -u b/core/includes/module.inc b/core/includes/module.inc
--- b/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -45,48 +45,47 @@
  *
  * 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
+ * @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 $fixed_list passed in.
+ *   Primarily intended for internal use (e.g., in install.php and update.php).
  *
  * @return
  *   An associative array whose keys and values are the names of the modules in
  *   the list.
  */
-function module_list($type = 'module_enabled', $fixed_list = NULL) {
+function module_list($type = 'module_enabled', array $fixed_list = NULL) {
   // Tracks data acquired from $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;
@@ -212,6 +211,7 @@
  */
 function system_list_reset() {
   drupal_static_reset('system_list');
+  drupal_static_reset('module_list');
   drupal_static_reset('system_rebuild_module_data');
   drupal_static_reset('list_themes');
   cache('bootstrap')->deleteMultiple(array('bootstrap_modules', 'system_list'));
@@ -660,8 +660,7 @@
     $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.
diff -u b/core/includes/schema.inc b/core/includes/schema.inc
--- b/core/includes/schema.inc
+++ b/core/includes/schema.inc
@@ -74,6 +74,7 @@
         // that it contains the complete list of modules before we go on to call
         // module_load_all_includes().
         drupal_static_reset('system_list');
+        drupal_static_reset('module_list');
         module_load_all_includes('install');
       }
 
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
@@ -776,6 +776,7 @@
 
     // Reload module list and implementations to ensure that test module hooks
     // aren't called after tests.
+    drupal_static_reset('system_list');
     drupal_static_reset('module_list');
     module_implements_reset();
 
diff -u b/core/modules/system/lib/Drupal/system/Tests/Module/ClassLoaderTest.php b/core/modules/system/lib/Drupal/system/Tests/Module/ClassLoaderTest.php
--- b/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 @@
  * Tests class loading.
  */
 class ClassLoaderTest extends WebTestBase {
-  protected $profile = 'testing';
-
   public static function getInfo() {
     return array(
       'name' => 'Module class loader',
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');
+  drupal_static_reset('module_list');
   module_implements_reset();
 
   // Load system theme data appropriately.
diff -u b/core/modules/system/system.test b/core/modules/system/system.test
--- b/core/modules/system/system.test
+++ b/core/modules/system/system.test
@@ -148,6 +148,7 @@
    */
   function assertModules(array $modules, $enabled) {
     drupal_static_reset('system_list');
+    drupal_static_reset('module_list');
     foreach ($modules as $module) {
       if ($enabled) {
         $message = 'Module "@module" is enabled.';
@@ -1674,6 +1675,7 @@
     $this->drupalPost('admin/modules', $edit, t('Save configuration'));
     $this->assertText(t('The configuration options have been saved.'), t('Modules status has been updated.'));
     drupal_static_reset('system_list');
+    drupal_static_reset('module_list');
     $this->assertFalse(module_exists('block'), t('Block module disabled.'));
 
     // At this point, no region is filled and fallback should be triggered.
@@ -1708,6 +1710,7 @@
     $this->drupalPost('admin/modules', $edit, t('Save configuration'));
     $this->assertText(t('The configuration options have been saved.'), t('Modules status has been updated.'));
     drupal_static_reset('system_list');
+    drupal_static_reset('module_list');
     $this->assertTrue(module_exists('block'), t('Block module re-enabled.'));
   }
 }