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

runtime/lib/mirrors_impl.dart

Index: runtime/lib/mirrors_impl.dart
===================================================================
--- runtime/lib/mirrors_impl.dart	(revision 24540)
+++ runtime/lib/mirrors_impl.dart	(working copy)
@@ -156,7 +156,7 @@
 
 // A VMReference is used to hold a reference to a VM-internal object,
 // which can include things like libraries, classes, etc.
-class VMReference extends NativeFieldWrapperClass1 {
+class OldVMReference extends NativeFieldWrapperClass1 {
 }
 
 abstract class _LocalVMObjectMirrorImpl extends _LocalMirrorImpl {
@@ -166,7 +166,7 @@
   // storing the Object reference itself here if the object is a Dart
   // language objects (except for objects of type VMReference, of
   // course).
-  VMReference _reference;
+  OldVMReference _reference;
 }
 
 abstract class _LocalObjectMirrorImpl extends _LocalVMObjectMirrorImpl