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

vendor/phpunit/phpunit-mock-objects/src/Framework/MockObject/Stub.php

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+<?php
+/*
+ * This file is part of the PHPUnit_MockObject package.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+/**
+ * An object that stubs the process of a normal method for a mock object.
+ *
+ * The stub object will replace the code for the stubbed method and return a
+ * specific value instead of the original value.
+ *
+ * @since Interface available since Release 1.0.0
+ */
+interface PHPUnit_Framework_MockObject_Stub extends PHPUnit_Framework_SelfDescribing
+{
+    /**
+     * Fakes the processing of the invocation $invocation by returning a
+     * specific value.
+     *
+     * @param  PHPUnit_Framework_MockObject_Invocation $invocation
+     *                                                             The invocation which was mocked and matched by the current method
+     *                                                             and argument matchers.
+     * @return mixed
+     */
+    public function invoke(PHPUnit_Framework_MockObject_Invocation $invocation);
+}