Add a VM defined class MirrorReference as an opaque pointer for Dart code to VM internal objects.

runtime/lib/mirror_reference.dart

Index: runtime/lib/mirror_reference.dart
===================================================================
--- runtime/lib/mirror_reference.dart	(revision 0)
+++ runtime/lib/mirror_reference.dart	(revision 0)
@@ -0,0 +1,10 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class _MirrorReference {
+  factory _MirrorReference._uninstantiable() {
+    throw new UnsupportedError(
+        "class _MirrorReference cannot be instantiated");
+  }
+}