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

runtime/vm/snapshot.h

 // 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.
 
 #ifndef VM_SNAPSHOT_H_
 #define VM_SNAPSHOT_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
@@ skipping 341 matching lines @@
   friend class LiteralToken;
   friend class PatchClass;
   friend class Script;
   friend class Stacktrace;
   friend class TokenStream;
   friend class Type;
   friend class TypeArguments;
   friend class TypeParameter;
   friend class UnresolvedClass;
   friend class WeakProperty;
+  friend class VMReference;
   DISALLOW_COPY_AND_ASSIGN(SnapshotReader);
 };
 
 
 class BaseWriter {
  public:
   // Size of the snapshot.
   intptr_t BytesWritten() const { return stream_.bytes_written(); }
 
   // Writes raw data to the stream (basic type).
@@ skipping 238 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_