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

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

@@ -0,0 +1,15 @@
+<?php
+class ClassThatImplementsSerializable implements Serializable
+{
+    public function serialize()
+    {
+        return get_object_vars($this);
+    }
+
+    public function unserialize($serialized)
+    {
+        foreach (unserialize($serialized) as $key => $value) {
+            $this->{$key} = $value;
+        }
+    }
+}