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

runtime/vm/object_graph.h

Index: runtime/vm/object_graph.h
diff --git a/runtime/vm/object_graph.h b/runtime/vm/object_graph.h
index 5187d5e2d8b6b19270095a21867a4ddce42d0dc3..70091d639bd1f394624602e8aa01a8f971da67e4 100644
--- a/runtime/vm/object_graph.h
+++ b/runtime/vm/object_graph.h
@@ -94,8 +94,10 @@ class ObjectGraph : public StackResource {
 
   // 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);