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 +1,7 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file

[siva, 2013/07/02 01:07:11]

2013

+// 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 {
+  _MirrorReference._privateConstructor();
+}

[siva, 2013/07/02 01:07:11]

I am assuming you will always create this object only from native code and do
not want it instantiated from Dart code.
You could use the model that bool.dart uses:

class bool {
  factory bool._uninstantiable() {
    throw new UnsupportedError(
        "class bool cannot be instantiated");
  }
}