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

vendor/symfony/http-foundation/Session/Storage/Handler/NativeSessionHandler.php

@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\HttpFoundation\Session\Storage\Handler;
+
+// Adds SessionHandler functionality if available.
+// @see http://php.net/sessionhandler
+if (PHP_VERSION_ID >= 50400) {
+    class NativeSessionHandler extends \SessionHandler
+    {
+    }
+} else {
+    class NativeSessionHandler
+    {
+    }
+}