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

vendor/guzzlehttp/promises/tests/RejectionExceptionTest.php

@@ -0,0 +1,47 @@
+<?php
+namespace GuzzleHttp\Promise\Tests;
+
+use GuzzleHttp\Promise\RejectionException;
+
+class Thing1
+{
+    public function __construct($message)
+    {
+        $this->message = $message;
+    }
+
+    public function __toString()
+    {
+        return $this->message;
+    }
+}
+
+class Thing2 implements \JsonSerializable
+{
+    public function jsonSerialize()
+    {
+        return '{}';
+    }
+}
+
+/**
+ * @covers GuzzleHttp\Promise\RejectionException
+ */
+class RejectionExceptionTest extends \PHPUnit_Framework_TestCase
+{
+    public function testCanGetReasonFromException()
+    {
+        $thing = new Thing1('foo');
+        $e = new RejectionException($thing);
+
+        $this->assertSame($thing, $e->getReason());
+        $this->assertEquals('The promise was rejected with reason: foo', $e->getMessage());
+    }
+
+    public function testCanGetReasonMessageFromJson()
+    {
+        $reason = new Thing2();
+        $e = new RejectionException($reason);
+        $this->assertContains("{}", $e->getMessage());
+    }
+}