5% smaller snapshots by omitting object ids when possible

runtime/vm/snapshot_ids.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_IDS_H_
 #define VM_SNAPSHOT_IDS_H_
 
 #include "vm/raw_object.h"
 
 namespace dart {
 
 // Index for predefined singleton objects used in a snapshot.
 enum {
   kNullObject = 0,
   kSentinelObject,
   kEmptyArrayObject,
   kZeroArrayObject,
   kTrueValue,
   kFalseValue,
   // Marker for special encoding of double objects in message snapshots.
   kDoubleObject,
+  // Object id has been optimized away; reader should use next available id.
+  kOmittedObjectId,
 
   kClassIdsOffset = kDoubleObject,
 
   // The class ids of predefined classes are included in this list
   // at an offset of kClassIdsOffset.
 
   kObjectType = (kNumPredefinedCids + kClassIdsOffset),
   kNullType,
   kDynamicType,
   kVoidType,
   kFunctionType,
   kNumberType,
   kSmiType,
   kMintType,
   kDoubleType,
   kIntType,
   kBoolType,
   kStringType,
   kArrayType,
 
   kInstanceObjectId,
   kMaxPredefinedObjectIds,
   kInvalidIndex = -1,
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_IDS_H_