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

vendor/guzzlehttp/promises/tests/AggregateExceptionTest.php

@@ -0,0 +1,14 @@
+<?php
+namespace GuzzleHttp\Promise\Tests;
+
+use GuzzleHttp\Promise\AggregateException;
+
+class AggregateExceptionTest extends \PHPUnit_Framework_TestCase
+{
+    public function testHasReason()
+    {
+        $e = new AggregateException('foo', ['baz', 'bar']);
+        $this->assertContains('foo', $e->getMessage());
+        $this->assertEquals(['baz', 'bar'], $e->getReason());
+    }
+}