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

vendor/phpunit/phpunit-mock-objects/tests/_fixture/Mockable.php

@@ -0,0 +1,28 @@
+<?php
+class Mockable
+{
+    public $constructorArgs;
+    public $cloned;
+
+    public function __construct($arg1 = null, $arg2 = null)
+    {
+        $this->constructorArgs = array($arg1, $arg2);
+    }
+
+    public function mockableMethod()
+    {
+        // something different from NULL
+        return true;
+    }
+
+    public function anotherMockableMethod()
+    {
+        // something different from NULL
+        return true;
+    }
+
+    public function __clone()
+    {
+        $this->cloned = true;
+    }
+}