diff --git a/core/lib/Drupal/Component/Plugin/PluginDefinition.php b/core/lib/Drupal/Component/Plugin/PluginDefinition.php
index 69c123d..32682b9 100644
--- a/core/lib/Drupal/Component/Plugin/PluginDefinition.php
+++ b/core/lib/Drupal/Component/Plugin/PluginDefinition.php
@@ -73,8 +73,8 @@ public function getId() {
    * {@inheritdoc}
    */
   public function setClass($class) {
-    if (!is_subclass_of($class, 'Drupal\Component\Plugin\PluginInspectionInterface')) {
-      throw new \InvalidArgumentException('Plugin classes must implement \Drupal\Component\Plugin\PluginInspectionInterface.');
+    if (!class_exists($class)) {
+      throw new \InvalidArgumentException(sprintf('Class %s does not exist.', $class));
     }
 
     $this->class = $class;
diff --git a/core/lib/Drupal/Component/Plugin/PluginDefinitionInterface.php b/core/lib/Drupal/Component/Plugin/PluginDefinitionInterface.php
index f1d1e60..91eadef 100644
--- a/core/lib/Drupal/Component/Plugin/PluginDefinitionInterface.php
+++ b/core/lib/Drupal/Component/Plugin/PluginDefinitionInterface.php
@@ -26,6 +26,7 @@
    * Sets the plugin ID.
    *
    * @param string $id
+   *   The plugin ID.
    *
    * @return $this
    */
@@ -35,6 +36,7 @@ public function setId($id);
    * Gets the plugin ID.
    *
    * @return string
+   *   The plugin ID.
    */
   public function getId();
 
@@ -42,8 +44,7 @@ public function getId();
    * Sets the class.
    *
    * @param string $class
-   *   A class that implements
-   *   \Drupal\Component\Plugin\PluginInspectionInterface.
+   *   A fully qualified class name.
    *
    * @return $this
    *
@@ -55,8 +56,7 @@ public function setClass($class);
    * Gets the class.
    *
    * @return string
-   *   A class that implements
-   *   \Drupal\Component\Plugin\PluginInspectionInterface.
+   *   A fully qualified class name.
    */
   public function getClass();
 
@@ -67,6 +67,7 @@ public function getClass();
    * package/component/module/extension that provides the plugin.
    *
    * @param string $provider
+   *   The provider.
    *
    * @return $this
    */
@@ -79,6 +80,7 @@ public function setProvider($provider);
    * package/component/module/extension that provides the plugin.
    *
    * @return string
+   *   The provider.
    */
   public function getProvider();
 
@@ -86,6 +88,7 @@ public function getProvider();
    * Sets the human-readable plugin label.
    *
    * @param string $label
+   *   The label.
    *
    * @return $this
    */
@@ -95,6 +98,7 @@ public function setLabel($label);
    * Gets the human-readable plugin label.
    *
    * @return string|null
+   *   The label or NULL if there is none.
    */
   public function getLabel();
 
@@ -102,7 +106,7 @@ public function getLabel();
    * Sets the deriver class.
    *
    * @param string $class
-   *   A class that implements
+   *   The fully qualified name of a class that implements
    *   \Drupal\Component\Plugin\Derivative\DeriverInterface.
    *
    * @return $this
@@ -115,7 +119,7 @@ public function setDeriverClass($class);
    * Gets the deriver class.
    *
    * @return string|null
-   *   A class that implements
+   *   The fully qualified name of a class that implements
    *   \Drupal\Component\Plugin\Derivative\DeriverInterface or null.
    */
   public function getDeriverClass();
diff --git a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
index 59839e2..615b52b 100644
--- a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
@@ -585,7 +585,6 @@ public function getRevisionTable();
   public function getDataTable();
 
   /**
-<<<<<<< HEAD
    * Gets the human-readable name of the entity type.
    *
    * @return string
@@ -595,9 +594,6 @@ public function getLabel();
 
   /**
    * Gets the lowercase form of the human-readable entity type name.
-=======
-   * Returns the lowercase form of the human-readable entity type name.
->>>>>>> Comment 1
    *
    * @return string
    *   The lowercase form of the human-readable entity type name.