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

vendor/behat/mink-browserkit-driver/tests/app.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace app;
+
+$app = new \Silex\Application();
+$app->register(new \Silex\Provider\SessionServiceProvider());
+
+$def = realpath(__DIR__.'/../vendor/behat/mink/driver-testsuite/web-fixtures');
+$ovr = realpath(__DIR__.'/web-fixtures');
+$cbk = function ($file) use ($app, $def, $ovr) {
+    $file = str_replace('.file', '.php', $file);
+    $path = file_exists($ovr.'/'.$file) ? $ovr.'/'.$file : $def.'/'.$file;
+    $resp = null;
+
+    ob_start();
+    include($path);
+    $content = ob_get_clean();
+
+    if ($resp) {
+        if ('' === $resp->getContent()) {
+            $resp->setContent($content);
+        }
+
+        return $resp;
+    }
+
+    return $content;
+};
+
+$app->get('/{file}', $cbk)->assert('file', '.*');
+$app->post('/{file}', $cbk)->assert('file', '.*');
+
+$app['debug'] = true;
+$app['exception_handler']->disable();
+$app['session.test'] = true;
+
+return $app;