Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/guzzlehttp/promises/tests/Thennable.php

@@ -0,0 +1,24 @@
+<?php
+namespace GuzzleHttp\Promise\Tests;
+
+use GuzzleHttp\Promise\Promise;
+
+class Thennable
+{
+    private $nextPromise = null;
+
+    public function __construct()
+    {
+        $this->nextPromise = new Promise();
+    }
+
+    public function then(callable $res = null, callable $rej = null)
+    {
+        return $this->nextPromise->then($res, $rej);
+    }
+
+    public function resolve($value)
+    {
+        $this->nextPromise->resolve($value);
+    }
+}