Fix for issue 23647 (https://github.com/dart-lang/sdk/issues/23647)

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 608 matching lines @@
     DISALLOW_COPY_AND_ASSIGN(Node);
   };
 
   Node* NodeForObjectId(intptr_t object_id) const {
     return nodes_[object_id - first_object_id_];
   }
 
   // Returns the id for the added object.
   intptr_t MarkAndAddObject(RawObject* raw, SerializeState state);
 
+  // Returns the id for the added object without marking it.
+  intptr_t AddObject(RawObject* raw, SerializeState state);
+
+  // Returns the id for the object it it exists in the list.
+  intptr_t FindObject(RawObject* raw);
+
   // Exhaustively processes all unserialized objects in this list. 'writer' may
   // concurrently add more objects.
   void SerializeAll(ObjectVisitor* writer);
 
   // Restores the tags of all objects in this list.
   void UnmarkAll() const;
 
  private:
   intptr_t first_object_id() const { return first_object_id_; }
   intptr_t next_object_id() const { return nodes_.length() + first_object_id_; }
@@ skipping 223 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_