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

vendor/symfony/dependency-injection/Tests/Fixtures/includes/ProjectExtension.php

@@ -0,0 +1,40 @@
+<?php
+
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\DependencyInjection\Extension\ExtensionInterface;
+
+class ProjectExtension implements ExtensionInterface
+{
+    public function load(array $configs, ContainerBuilder $configuration)
+    {
+        $config = call_user_func_array('array_merge', $configs);
+
+        $configuration->setDefinition('project.service.bar', new Definition('FooClass'));
+        $configuration->setParameter('project.parameter.bar', isset($config['foo']) ? $config['foo'] : 'foobar');
+
+        $configuration->setDefinition('project.service.foo', new Definition('FooClass'));
+        $configuration->setParameter('project.parameter.foo', isset($config['foo']) ? $config['foo'] : 'foobar');
+
+        return $configuration;
+    }
+
+    public function getXsdValidationBasePath()
+    {
+        return false;
+    }
+
+    public function getNamespace()
+    {
+        return 'http://www.example.com/schema/project';
+    }
+
+    public function getAlias()
+    {
+        return 'project';
+    }
+
+    public function getConfiguration(array $config, ContainerBuilder $container)
+    {
+    }
+}