Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.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 RUNTIME_VM_SCAVENGER_H_
 #define RUNTIME_VM_SCAVENGER_H_
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/dart.h"
@@ skipping 176 matching lines @@
 
   intptr_t collections() const { return collections_; }
 
 #ifndef PRODUCT
   void PrintToJSONObject(JSONObject* object) const;
 #endif  // !PRODUCT
 
   void AllocateExternal(intptr_t size);
   void FreeExternal(intptr_t size);
 
+  void UnmarkNewSpace();
+
  private:
   // Ids for time and data records in Heap::GCStats.
   enum {
     // Time
     kDummyScavengeTime = 0,
     kSafePoint = 1,
     kVisitIsolateRoots = 2,
     kIterateStoreBuffers = 3,
     kProcessToSpace = 4,
     kIterateWeaks = 5,
@@ skipping 90 matching lines @@
 
   friend class ScavengerVisitor;
   friend class ScavengerWeakVisitor;
 
   DISALLOW_COPY_AND_ASSIGN(Scavenger);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_SCAVENGER_H_