Implement mirror equality on the VM. Resolves issue 12026.

runtime/lib/mirror_reference.dart

 // 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");
   }
+
+  bool operator ==(other) native "MirrorReference_equals";
 }