Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/modules/rest/src/ResourceResponseTrait.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\rest;
+
+
+trait ResourceResponseTrait {
+
+  /**
+   * Response data that should be serialized.
+   *
+   * @var mixed
+   */
+  protected $responseData;
+
+  /**
+   * Returns response data that should be serialized.
+   *
+   * @return mixed
+   *   Response data that should be serialized.
+   */
+  public function getResponseData() {
+    return $this->responseData;
+  }
+
+}