Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/vendor/symfony/dependency-injection/IntrospectableContainerInterface.php

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+<?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\DependencyInjection;
+
+/**
+ * IntrospectableContainerInterface defines additional introspection functionality
+ * for containers, allowing logic to be implemented based on a Container's state.
+ *
+ * @author Evan Villemez <evillemez@gmail.com>
+ */
+interface IntrospectableContainerInterface extends ContainerInterface
+{
+    /**
+     * Check for whether or not a service has been initialized.
+     *
+     * @param string $id
+     *
+     * @return bool true if the service has been initialized, false otherwise
+     */
+    public function initialized($id);
+}