Update to Drupal 8.2.5. For more information, see https://www.drupal.org/project/drupal/releases/8.2.5

core/modules/big_pipe/src/Render/BigPipeInterface.php

@@ -138,6 +138,12 @@ interface BigPipeInterface {
    *   The HTML response content to send.
    * @param array $attachments
    *   The HTML response's attachments.
+   *
+   * @internal
+   *   This method should only be invoked by
+   *   \Drupal\big_pipe\Render\BigPipeResponse, which is itself an internal
+   *   class. Furthermore, the signature of this method will change in
+   *   https://www.drupal.org/node/2657684.
    */
   public function sendContent($content, array $attachments);
 

core/modules/big_pipe/src/Render/BigPipeResponse.php

@@ -13,7 +13,10 @@ use Drupal\Core\Render\HtmlResponse;
  *
  * @see \Drupal\big_pipe\Render\BigPipeInterface
  *
- * @todo Will become obsolete with https://www.drupal.org/node/2577631
+ * @internal
+ *   This is a temporary solution until a generic response emitter interface is
+ *   created in https://www.drupal.org/node/2577631. Only code internal to
+ *   BigPipe should instantiate or type hint to this class.
  */
 class BigPipeResponse extends HtmlResponse {