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

core/vendor/symfony/dependency-injection/Parameter.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;
+
+/**
+ * Parameter represents a parameter reference.
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ *
+ * @api
+ */
+class Parameter
+{
+    private $id;
+
+    /**
+     * Constructor.
+     *
+     * @param string $id The parameter key
+     */
+    public function __construct($id)
+    {
+        $this->id = $id;
+    }
+
+    /**
+     * __toString.
+     *
+     * @return string The parameter key
+     */
+    public function __toString()
+    {
+        return (string) $this->id;
+    }
+}