Implement weak references sets and provide an embedding API.

runtime/vm/gc_marker.h

 // Copyright (c) 2011, 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_GC_MARKER_H_
 #define VM_GC_MARKER_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
@@ skipping 12 matching lines @@
  public:
   explicit GCMarker(Heap* heap) : heap_(heap) { }
   ~GCMarker() { }
 
   void MarkObjects(Isolate* isolate, PageSpace* page_space);
 
  private:
   void Prologue(Isolate* isolate);
   void IterateRoots(Isolate* isolate, ObjectPointerVisitor* visitor);
   void IterateWeakRoots(Isolate* isolate, HandleVisitor* visitor);
+  void IterateWeakReferences(Isolate* isolate, MarkingVisitor* visitor);
   void DrainMarkingStack(Isolate* isolate, MarkingVisitor* visitor);
 
   Heap* heap_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(GCMarker);
 };
 
 }  // namespace dart
 
 #endif  // VM_GC_MARKER_H_