Enable iterating over roots from helper threads by moving stack walking.

runtime/vm/pages.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_PAGES_H_
 #define VM_PAGES_H_
 
 #include "vm/freelist.h"
 #include "vm/globals.h"
 #include "vm/lockers.h"
@@ skipping 409 matching lines @@
   intptr_t max_capacity_in_words_;
   intptr_t max_external_in_words_;
   // NOTE: The capacity component of usage_ is updated by the concurrent
   // sweeper. Use (Increase)CapacityInWords(Locked) for thread-safe access.
   SpaceUsage usage_;
 
   // Keep track of running MarkSweep tasks.
   Monitor* tasks_lock_;
   intptr_t tasks_;
 #if defined(DEBUG)
-  bool is_iterating_;
+  Thread* iterating_thread_;
 #endif
   PageSpaceController page_space_controller_;
 
   int64_t gc_time_micros_;
   intptr_t collections_;
 
   friend class ExclusivePageIterator;
   friend class ExclusiveCodePageIterator;
   friend class ExclusiveLargePageIterator;
   friend class HeapIterationScope;
   friend class PageSpaceController;
   friend class SweeperTask;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(PageSpace);
 };
 
 }  // namespace dart
 
 #endif  // VM_PAGES_H_