5% smaller snapshots by omitting object ids when possible

runtime/vm/dart_api_message.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_DART_API_MESSAGE_H_
 #define VM_DART_API_MESSAGE_H_
 
 #include "include/dart_native_api.h"
 #include "vm/dart_api_state.h"
 #include "vm/snapshot.h"
@@ skipping 102 matching lines @@
   Dart_CObject* ReadVMSymbol(intptr_t object_id);
   Dart_CObject* ReadObjectRef();
   Dart_CObject* ReadObject();
 
   // Add object to backward references.
   void AddBackRef(intptr_t id, Dart_CObject* obj, DeserializeState state);
 
   // Get an object from the backward references list.
   Dart_CObject* GetBackRef(intptr_t id);
 
+  intptr_t NextAvailableObjectId() const;
+
   Dart_CObject_Internal* AsInternal(Dart_CObject* object) {
     ASSERT(object->type >= Dart_CObject_kNumberOfTypes);
     return reinterpret_cast<Dart_CObject_Internal*>(object);
   }
 
   // Allocation of the structures for the decoded message happens
   // either in the supplied zone or using the supplied allocation
   // function.
   ReAlloc alloc_;
   ApiGrowableArray<BackRefNode*> backward_references_;
@@ skipping 51 matching lines @@
   Dart_CObject** forward_list_;
   intptr_t forward_list_length_;
   intptr_t forward_id_;
 
   DISALLOW_COPY_AND_ASSIGN(ApiMessageWriter);
 };
 
 }  // namespace dart
 
 #endif  // VM_DART_API_MESSAGE_H_