Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/vendor/guzzlehttp/promises/src/functions.php

@@ -1,11 +1,6 @@
 <?php
 namespace GuzzleHttp\Promise;
 
-// Don't redefine the functions if included multiple times.
-if (function_exists('GuzzleHttp\Promise\promise_for')) {
-    return;
-}
-
 /**
  * Get the global task queue used for promise resolution.
  *

core/vendor/guzzlehttp/promises/src/functions_include.php

@@ -0,0 +1,6 @@
+<?php
+
+// Don't redefine the functions if included multiple times.
+if (!function_exists('GuzzleHttp\Promise\promise_for')) {
+    require __DIR__ . '/functions.php';
+}