diff --git a/core/lib/Drupal/Component/Plugin/Definition/PluginDefinitionInterface.php b/core/lib/Drupal/Component/Plugin/Definition/PluginDefinitionInterface.php
new file mode 100644
index 0000000..1132bb5
--- /dev/null
+++ b/core/lib/Drupal/Component/Plugin/Definition/PluginDefinitionInterface.php
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Plugin\PluginDefinitionInterface.
+ */
+
+namespace Drupal\Component\Plugin\Definition;
+
+/**
+ * Defines a plugin definition.
+ *
+ * @ingroup Plugin
+ */
+interface PluginDefinitionInterface {
+
+  /**
+   * Sets the class.
+   *
+   * @param string $class
+   *   A fully qualified class name.
+   *
+   * @return $this
+   *
+   * @throws \InvalidArgumentException
+   *   If the class is invalid.
+   */
+  public function setClass($class);
+
+  /**
+   * Gets the class.
+   *
+   * @return string
+   *   A fully qualified class name.
+   */
+  public function getClass();
+
+}
diff --git a/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php b/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php
index 9c45148..de1d43a 100644
--- a/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php
+++ b/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php
@@ -6,6 +6,7 @@
 
 namespace Drupal\Component\Plugin\Factory;
 
+use Drupal\Component\Plugin\Definition\PluginDefinitionInterface;
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
 use Drupal\Component\Plugin\Exception\PluginException;
 
@@ -63,7 +64,7 @@ public function createInstance($plugin_id, array $configuration = array()) {
    *
    * @param string $plugin_id
    *   The id of a plugin.
-   * @param mixed $plugin_definition
+   * @param \Drupal\Component\Plugin\Definition\PluginDefinitionInterface|mixed[] $plugin_definition
    *   The plugin definition associated with the plugin ID.
    * @param string $required_interface
    *   (optional) THe required plugin interface.
@@ -85,7 +86,7 @@ public static function getPluginClass($plugin_id, $plugin_definition = NULL, $re
 
       $class = $plugin_definition['class'];
     }
-    elseif (is_object($plugin_definition) && method_exists($plugin_definition, 'getClass')) {
+    elseif ($plugin_definition instanceof PluginDefinitionInterface) {
       if (!$plugin_definition->getClass()) {
         throw new PluginException($missing_class_message);
       }
@@ -100,8 +101,8 @@ public static function getPluginClass($plugin_id, $plugin_definition = NULL, $re
       throw new PluginException(sprintf('Plugin (%s) instance class "%s" does not exist.', $plugin_id, $class));
     }
 
-    if ($required_interface && !is_subclass_of($plugin_definition['class'], $required_interface)) {
-      throw new PluginException(sprintf('Plugin "%s" (%s) must implement interface %s.', $plugin_id, $plugin_definition['class'], $required_interface));
+    if ($required_interface && !is_subclass_of($class, $required_interface)) {
+      throw new PluginException(sprintf('Plugin "%s" (%s) must implement interface %s.', $plugin_id, $class, $required_interface));
     }
 
     return $class;
diff --git a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
index 1a3805e..731644c 100644
--- a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
@@ -6,6 +6,7 @@
  */
 
 namespace Drupal\Core\Entity;
+use Drupal\Component\Plugin\Definition\PluginDefinitionInterface;
 
 /**
  * Provides an interface for an entity type and its metadata.
@@ -15,7 +16,7 @@
  * implemented to alter existing data and fill-in defaults. Module-specific
  * properties should be documented in the hook implementations defining them.
  */
-interface EntityTypeInterface {
+interface EntityTypeInterface extends PluginDefinitionInterface {
 
   /**
    * The maximum length of ID, in characters.
@@ -67,14 +68,6 @@ public function id();
   public function getProvider();
 
   /**
-   * Gets the name of the entity type class.
-   *
-   * @return string
-   *   The name of the entity type class.
-   */
-  public function getClass();
-
-  /**
    * Gets the name of the original entity type class.
    *
    * In case the class name was changed with setClass(), this will return