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

runtime/vm/object_graph.cc

 // 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.
 
 #include "vm/object_graph.h"
 
 #include "vm/dart.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
@@ skipping 493 matching lines @@
 
   intptr_t count() const { return count_; }
 
  private:
   WriteStream* stream_;
   WritePointerVisitor ptr_writer_;
   intptr_t count_;
 };
 
 
-intptr_t ObjectGraph::Serialize(WriteStream* stream) {
+intptr_t ObjectGraph::Serialize(WriteStream* stream, bool collect_garbage) {
+  if (collect_garbage) {
+    isolate()->heap()->CollectAllGarbage();
+  }
   // Current encoding assumes objects do not move, so promote everything to old.
   isolate()->heap()->new_space()->Evacuate();

[Ivan Posva, 2016/03/18 23:38:58]

You could also run a scavenge followed by the call to CollectAllGarbage above.
This should have the same effect as evacuating after the full collection with
the additional benefit of collecting new-old-new cycles.

+
   WriteGraphVisitor visitor(isolate(), stream);
   stream->WriteUnsigned(kObjectAlignment);
   stream->WriteUnsigned(0);
   stream->WriteUnsigned(0);
   stream->WriteUnsigned(0);
   {
     WritePointerVisitor ptr_writer(isolate(), stream);
     isolate()->IterateObjectPointers(&ptr_writer, false);
   }
   stream->WriteUnsigned(0);
   IterateObjects(&visitor);
   return visitor.count() + 1;  // + root
 }
 
 }  // namespace dart