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

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
===================================================================
--- runtime/vm/raw_object.cc	(revision 24693)
+++ runtime/vm/raw_object.cc	(working copy)
@@ -807,4 +807,13 @@
   return WeakProperty::InstanceSize();
 }
 
+
+intptr_t RawMirrorReference::VisitMirrorReferencePointers(
+    RawMirrorReference* raw_obj, ObjectPointerVisitor* visitor) {
+  // Make sure that we got here with the tagged pointer as this.
+  ASSERT(raw_obj->IsHeapObject());
+  visitor->VisitPointers(raw_obj->from(), raw_obj->to());
+  return MirrorReference::InstanceSize();
+}
+
 }  // namespace dart