Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/lib/Drupal/Component/Plugin/FallbackPluginManagerInterface.php

@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Plugin\FallbackPluginManagerInterface.
+ */
+
+namespace Drupal\Component\Plugin;
+
+/**
+ * An interface implemented by plugin managers with fallback plugin behaviors.
+ */
+interface FallbackPluginManagerInterface {
+
+  /**
+   * Gets a fallback id for a missing plugin.
+   *
+   * @param string $plugin_id
+   *   The ID of the missing requested plugin.
+   * @param array $configuration
+   *   An array of configuration relevant to the plugin instance.
+   *
+   * @return string
+   *   The id of an existing plugin to use when the plugin does not exist.
+   */
+  public function getFallbackPluginId($plugin_id, array $configuration = array());
+
+}