Issue #2226437 by fago, tstoeckler, mauzeh: Create ContentEntityTypeInterface.

core/lib/Drupal/Core/Entity/ContentEntityType.php

@@ -10,7 +10,7 @@
 /**
  * Defines a config entity type annotation object.
  */
-class ContentEntityType extends EntityType {
+class ContentEntityType extends EntityType implements ContentEntityTypeInterface {
 
   /**
    * {@inheritdoc}

core/lib/Drupal/Core/Entity/ContentEntityTypeInterface.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Entity\ContentEntityTypeInterface.
+ */
+
+namespace Drupal\Core\Entity;
+
+/**
+ * Provides an interface for a content entity type.
+ */
+interface ContentEntityTypeInterface {
+
+}