diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index b8c38ce..823bd60 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -235,9 +235,7 @@ public function getDefinitions() {
       return $cache->data;
     }
     else {
-      // @todo Remove array_filter() once http://drupal.org/node/1780396 is
-      //   resolved.
-      $definitions = array_filter(parent::getDefinitions());
+      $definitions = parent::getDefinitions();
       cache($this->cacheBin)->set($this->cacheKey, $definitions, $this->cacheExpire, $this->cacheTags);
       return $definitions;
     }
@@ -249,12 +247,6 @@ public function getDefinitions() {
   protected function processDefinition(&$definition, $plugin_id) {
     parent::processDefinition($definition, $plugin_id);
 
-    // @todo Remove this check once http://drupal.org/node/1780396 is resolved.
-    if (!module_exists($definition['module'])) {
-      $definition = NULL;
-      return;
-    }
-
     foreach ($definition['view_modes'] as $view_mode => $view_mode_info) {
       $definition['view_modes'][$view_mode] += array(
         'custom_settings' => FALSE,
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index a21d496..ec5b0f3 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -151,6 +151,11 @@ protected function enableModules(array $modules, $install = TRUE) {
     // Call module_enable() to enable (install) the new modules.
     if ($install) {
       module_enable($modules);
+      // Register the PSR-0 namespace of the enabled modules.
+      foreach ($modules as $module) {
+        // Register the PSR-0 namespace of the enabled modules.
+        drupal_classloader_register($module, dirname($this->moduleList[$module]['filename']));
+      }
     }
     // Otherwise, only ensure that the new modules are loaded.
     else {
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 2c47954..7d228cd 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -389,7 +389,7 @@ function simpletest_classloader_register() {
   $all_data += system_rebuild_theme_data();
   $loader = drupal_classloader();
   foreach ($all_data as $name => $data) {
-    $loader->registerNamespace('Drupal\\' . $name . '\Tests', DRUPAL_ROOT . '/' . $data->filename . '/lib');
+    $loader->registerNamespace('Drupal\\' . $name . '\Tests', DRUPAL_ROOT . '/' . dirname($data->filename) . '/lib');
   }
 }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php
index 41070c3..8a30531 100644
--- a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php
@@ -92,6 +92,7 @@ function testCompileDIC() {
       'bundle_test' => 'bundle_test',
     );
     $cache->flush();
+    drupal_classloader_register('bundle_test', dirname(drupal_get_filename('module', 'bundle_test')));
     $kernel = new DrupalKernel('testing', FALSE, $module_enabled, $cache);
     $kernel->boot();
     // Instantiate it a second time and we should still get a ContainerBuilder
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 ae9deb1..bb039bc 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
@@ -24,6 +24,9 @@ public static function getInfo() {
 
   public function setUp() {
     parent::setUp();
+
+    $this->enableModules(array('plugin_test'));
+
     $this->expectedDefinitions = array(
       'apple' => array(
         'id' => 'apple',
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/DiscoveryTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/DiscoveryTestBase.php
index e95d238..9c41bee 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/DiscoveryTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/DiscoveryTestBase.php
@@ -7,12 +7,12 @@
 
 namespace Drupal\system\Tests\Plugin\Discovery;
 
-use Drupal\simpletest\UnitTestBase;
+use Drupal\simpletest\DrupalUnitTestBase;
 
 /**
  * Tests that plugins are correctly discovered.
  */
-class DiscoveryTestBase extends UnitTestBase {
+class DiscoveryTestBase extends DrupalUnitTestBase {
 
   /**
    * The discovery component to test.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php
index 5db9322..a4538e7 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\system\Tests\Plugin;
 
-use Drupal\simpletest\UnitTestBase;
+use Drupal\simpletest\DrupalUnitTestBase;
 use Drupal\plugin_test\Plugin\TestPluginManager;
 use Drupal\plugin_test\Plugin\MockBlockManager;
 use Drupal\plugin_test\Plugin\DefaultsTestPluginManager;
@@ -15,7 +15,7 @@
 /**
  * Base class for Plugin API unit tests.
  */
-abstract class PluginTestBase extends UnitTestBase {
+abstract class PluginTestBase extends DrupalUnitTestBase {
   protected $testPluginManager;
   protected $testPluginExpectedDefinitions;
   protected $mockBlockManager;
@@ -26,6 +26,8 @@
   public function setUp() {
     parent::setUp();
 
+    $this->enableModules(array('plugin_test'));
+
     // Real modules implementing plugin types may expose a module-specific API
     // for retrieving each type's plugin manager, or make them available in
     // Drupal's dependency injection container, but for unit testing, we get
diff --git a/core/modules/system/lib/Drupal/system/Tests/Routing/ChainMatcherTest.php b/core/modules/system/lib/Drupal/system/Tests/Routing/ChainMatcherTest.php
index c6b28cc..9ca0b13 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Routing/ChainMatcherTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Routing/ChainMatcherTest.php
@@ -22,7 +22,7 @@
  * Basic tests for the ChainMatcher.
  */
 class ChainMatcherTest extends UnitTestBase {
-  
+
   public static function getInfo() {
     return array(
       'name' => 'Chain matcher tests',
@@ -31,6 +31,12 @@ public static function getInfo() {
     );
   }
 
+  public function setUp() {
+    parent::setUp();
+    // Make sure the system.module classes can be found.
+    drupal_classloader_register('system', dirname(drupal_get_filename('module', 'system')));
+  }
+
   /**
    * Confirms that the expected exception is thrown.
    */
diff --git a/core/modules/system/lib/Drupal/system/Tests/Routing/ControllerResolverTest.php b/core/modules/system/lib/Drupal/system/Tests/Routing/ControllerResolverTest.php
index 46ed2a3..d27b95b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Routing/ControllerResolverTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Routing/ControllerResolverTest.php
@@ -33,6 +33,9 @@ function testContainerAware() {
     $container = new Container();
     $resolver = new ControllerResolver($container);
 
+    // Make sure the system.module classes can be found.
+    drupal_classloader_register('system', dirname(drupal_get_filename('module', 'system')));
+
     $request = Request::create('/some/path');
     $request->attributes->set('_controller', '\Drupal\system\Tests\Routing\MockController::run');
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Routing/HttpMethodMatcherTest.php b/core/modules/system/lib/Drupal/system/Tests/Routing/HttpMethodMatcherTest.php
index c98da2e..e8221cb 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Routing/HttpMethodMatcherTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Routing/HttpMethodMatcherTest.php
@@ -39,12 +39,15 @@ public static function getInfo() {
     );
   }
 
-  function __construct($test_id = NULL) {
-    parent::__construct($test_id);
+  function setUp() {
+    parent::setUp();
+
+    // Make sure the system.module classes can be found.
+    drupal_classloader_register('system', dirname(drupal_get_filename('module', 'system')));
 
     $this->fixtures = new RoutingFixtures();
   }
-  
+
   /**
    * Confirms that the HttpMethod matcher matches properly.
    */
diff --git a/core/modules/system/lib/Drupal/system/Tests/Routing/NestedMatcherTest.php b/core/modules/system/lib/Drupal/system/Tests/Routing/NestedMatcherTest.php
index 444785c..f993d2a 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Routing/NestedMatcherTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Routing/NestedMatcherTest.php
@@ -39,8 +39,11 @@ public static function getInfo() {
     );
   }
 
-  function __construct($test_id = NULL) {
-    parent::__construct($test_id);
+  function setUp() {
+    parent::setUp();
+
+    // Make sure the system.module classes can be found.
+    drupal_classloader_register('system', dirname(drupal_get_filename('module', 'system')));
 
     $this->fixtures = new RoutingFixtures();
   }
diff --git a/core/modules/user/lib/Drupal/user/Tests/TempStoreDatabaseTest.php b/core/modules/user/lib/Drupal/user/Tests/TempStoreDatabaseTest.php
index 5fb54e3..06fd7f8 100644
--- a/core/modules/user/lib/Drupal/user/Tests/TempStoreDatabaseTest.php
+++ b/core/modules/user/lib/Drupal/user/Tests/TempStoreDatabaseTest.php
@@ -64,6 +64,8 @@ protected function setUp() {
     $schema = system_schema();
     db_create_table('semaphore', $schema['semaphore']);
     db_create_table('key_value_expire', $schema['key_value_expire']);
+    // Register the namespace of user.module.
+    drupal_classloader_register('user', dirname(drupal_get_filename('module', 'user')));
 
     // Create several objects for testing.
     for ($i = 0; $i <= 3; $i++) {