Perform a full GC by default when requesting a heap snapshot.

runtime/vm/object_graph.h

 // Copyright (c) 2014, 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_OBJECT_GRAPH_H_
 #define VM_OBJECT_GRAPH_H_
 
 #include "vm/allocation.h"
 #include "vm/object.h"
 
@@ skipping 76 matching lines @@
   // Find the objects that reference 'obj'. Populates the provided array with
   // pairs of (object pointing to 'obj', offset of pointer in words), as far as
   // there is room. Returns the number of objects found.
   //
   // An object for which this function answers no inbound references might still
   // be live due to references from the stack or embedder handles.
   intptr_t InboundReferences(Object* obj, const Array& references);
 
   // Write the isolate's object graph to 'stream'. Smis and nulls are omitted.
   // Returns the number of nodes in the stream, including the root.
+  // If collect_garabage is false, the graph will include weakly-reachable
+  // objects.
   // TODO(koda): Document format; support streaming/chunking.
-  intptr_t Serialize(WriteStream* stream);
+  intptr_t Serialize(WriteStream* stream, bool collect_garbage);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ObjectGraph);
 };
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_GRAPH_H_