Reclaim the CreatedFromSnapshot bit and use it to indicate VM Heap object

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 432 matching lines @@
                      const Array& result,
                      intptr_t len,
                      intptr_t tags);
 
   intptr_t NextAvailableObjectId() const;
 
   void SetReadException(const char* msg);
 
   RawObject* VmIsolateSnapshotObject(intptr_t index) const;
 
+  bool is_vm_isolate() const;
+
   Snapshot::Kind kind_;  // Indicates type of snapshot(full, script, message).
   Isolate* isolate_;  // Current isolate.
   Zone* zone_;  // Zone for allocations while reading snapshot.
   Heap* heap_;  // Heap of the current isolate.
   PageSpace* old_space_;  // Old space of the current isolate.
   Class& cls_;  // Temporary Class handle.
   Object& obj_;  // Temporary Object handle.
   PassiveObject& pobj_;  // Temporary PassiveObject handle.
   Array& array_;  // Temporary Array handle.
   Field& field_;  // Temporary Field handle.
@@ skipping 469 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_