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

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

@@ -0,0 +1,28 @@
+<?php
+
+class SingletonClass
+{
+    public static function getInstance()
+    {
+    }
+
+    public function doSomething()
+    {
+    }
+
+    protected function __construct()
+    {
+    }
+
+    final private function __sleep()
+    {
+    }
+
+    final private function __wakeup()
+    {
+    }
+
+    final private function __clone()
+    {
+    }
+}