Debug garbage collector does not correctly remove cross-gen garbage

runtime/vm/scavenger.cc

 // 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.
 
 #include "vm/scavenger.h"
 
 #include "vm/dart.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
@@ skipping 878 matching lines @@
   external_size_ += size;
 }
 
 
 void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ -= size;
   ASSERT(external_size_ >= 0);
 }
 
+void Scavenger::UnmarkNewSpace() {
+  uword cur = FirstObjectStart();
+  while (cur < top_) {
+    RawObject* raw_obj = RawObject::FromAddr(cur);
+    if (raw_obj->IsMarked() && !raw_obj->IsVMHeapObject()) {
+      raw_obj->ClearMarkBit();
+    }
+    cur += raw_obj->Size();
+  }
+}
+
 }  // namespace dart