5% smaller snapshots by omitting object ids when possible

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 327 matching lines @@
   RawObject* ReadIndexedObject(intptr_t object_id);
 
   // Read an inlined object from the stream.
   RawObject* ReadInlinedObject(intptr_t object_id);
 
   // Based on header field check to see if it is an internal VM class.
   RawClass* LookupInternalClass(intptr_t class_header);
 
   void ArrayReadFrom(const Array& result, intptr_t len, intptr_t tags);
 
+  intptr_t NextAvailableObjectId() const;
+
   Snapshot::Kind kind_;  // Indicates type of snapshot(full, script, message).
   Isolate* isolate_;  // Current isolate.
   Class& cls_;  // Temporary Class handle.
   Object& obj_;  // Temporary Object handle.
   Array& array_;  // Temporary Array handle.
   Field& field_;  // Temporary Field handle.
   String& str_;  // Temporary String handle.
   Library& library_;  // Temporary library handle.
   AbstractType& type_;  // Temporary type handle.
   TypeArguments& type_arguments_;  // Temporary type argument handle.
@@ skipping 340 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_