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

runtime/vm/raw_object_snapshot.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/bigint_operations.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/symbols.h"
 #include "vm/visitor.h"
@@ skipping 2628 matching lines @@
 
   // Write out the class and tags information.
   writer->WriteIndexedObject(kWeakPropertyCid);
   writer->WriteIntptrValue(writer->GetObjectTags(this));
 
   // Write out all the other fields.
   writer->Write<RawObject*>(ptr()->key_);
   writer->Write<RawObject*>(ptr()->value_);
 }
 
+
+RawMirrorReference* MirrorReference::ReadFrom(SnapshotReader* reader,
+                                          intptr_t object_id,
+                                          intptr_t tags,
+                                          Snapshot::Kind kind) {
+  UNREACHABLE();
+  return MirrorReference::null();
+}
+
+
+void RawMirrorReference::WriteTo(SnapshotWriter* writer,
+                                 intptr_t object_id,
+                                 Snapshot::Kind kind) {
+  UNREACHABLE();
+}
+
 }  // namespace dart