diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index eb5eeee..53a9b5b 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -259,7 +259,11 @@ protected function findDefinitions() {
    *
    * @throws \Drupal\Component\Plugin\Exception\PluginException
    */
-  protected function enforcePluginInterface($plugin_id, $plugin_definition) {
+  protected function enforcePluginInterface($plugin_id, array $plugin_definition) {
+    // Make it possible to not specify an interface.
+    if (!isset($this->interface)) {
+      return;
+    }
     // If there are multiple valid interfaces, check for at least one.
     if (is_array($this->interface) && array_intersect(class_implements($plugin_definition['class']), $this->interface)) {
       return;
diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManager.php b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
index 7889be1..c9afe86 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManager.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
@@ -395,11 +395,4 @@ public function clearCachedDefinitions() {
     $this->prototypes = array();
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function enforcePluginInterface($plugin_id, $plugin_definition) {
-    // Do not enforce an interface for typed data plugins.
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Validation/ConstraintManager.php b/core/lib/Drupal/Core/Validation/ConstraintManager.php
index 70adb95..a2b46d3 100644
--- a/core/lib/Drupal/Core/Validation/ConstraintManager.php
+++ b/core/lib/Drupal/Core/Validation/ConstraintManager.php
@@ -138,12 +138,4 @@ public function getDefinitionsByType($type) {
     }
     return $definitions;
   }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function enforcePluginInterface($plugin_id, $plugin_definition) {
-    // Do not enforce an interface for constraint plugins.
-  }
-
 }
diff --git a/core/modules/aggregator/src/Plugin/AggregatorPluginManager.php b/core/modules/aggregator/src/Plugin/AggregatorPluginManager.php
index 619e125..1e5d4b6 100644
--- a/core/modules/aggregator/src/Plugin/AggregatorPluginManager.php
+++ b/core/modules/aggregator/src/Plugin/AggregatorPluginManager.php
@@ -7,8 +7,6 @@
 
 namespace Drupal\aggregator\Plugin;
 
-use Drupal\Component\Plugin\Exception\PluginException;
-use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Plugin\DefaultPluginManager;
diff --git a/core/modules/migrate_drupal/src/Plugin/MigrateLoadInterface.php b/core/modules/migrate_drupal/src/Plugin/MigrateLoadInterface.php
index f5545e8..ce3ed83 100644
--- a/core/modules/migrate_drupal/src/Plugin/MigrateLoadInterface.php
+++ b/core/modules/migrate_drupal/src/Plugin/MigrateLoadInterface.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Contains Drupal\migrate\Plugin\MigrateLoadInterface
+ * Contains Drupal\migrate_drupal\Plugin\MigrateLoadInterface
  */
 
 namespace Drupal\migrate_drupal\Plugin;
diff --git a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
index c202cbc..4b3652c 100644
--- a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
@@ -254,6 +254,34 @@ public function testGetDefinitionsWithInvalidInterfaces() {
     $plugin_manager->getDefinitions();
   }
 
+  /**
+   * Tests plugins without a required interface.
+   *
+   * @covers ::getDefinitions
+   * @covers ::enforcePluginInterface
+   */
+  public function testGetDefinitionsWithoutRequiredInterface() {
+    $module_handler = $this->getMock('Drupal\Core\Extension\ModuleHandlerInterface');
+
+    $module_handler->expects($this->any())
+      ->method('moduleExists')
+      ->with('plugin_test')
+      ->willReturn(FALSE);
+
+    $this->expectedDefinitions['kale'] = array(
+      'id' => 'kale',
+      'label' => 'Kale',
+      'color' => 'green',
+      'class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Kale',
+      'provider' => 'plugin_test',
+    );
+    $this->expectedDefinitions['apple']['provider'] = 'plugin_test';
+    $this->expectedDefinitions['banana']['provider'] = 'plugin_test';
+
+    $plugin_manager = new TestPluginManager($this->namespaces, $this->expectedDefinitions, $module_handler, NULL);
+    $plugin_manager->getDefinitions();
+  }
+
 }
 
 if (!defined('DRUPAL_ROOT')) {