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

runtime/vm/pages.cc

 // 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.
 
 #include "vm/pages.h"
 
 #include "platform/assert.h"
 #include "vm/compiler_stats.h"
 #include "vm/gc_marker.h"
 #include "vm/gc_sweeper.h"
@@ skipping 143 matching lines @@
       exec_pages_(NULL),
       exec_pages_tail_(NULL),
       large_pages_(NULL),
       bump_top_(0),
       bump_end_(0),
       max_capacity_in_words_(max_capacity_in_words),
       max_external_in_words_(max_external_in_words),
       tasks_lock_(new Monitor()),
       tasks_(0),
 #if defined(DEBUG)
-      is_iterating_(false),
+      iterating_thread_(NULL),
 #endif
       page_space_controller_(heap,
                              FLAG_old_gen_growth_space_ratio,
                              FLAG_old_gen_growth_rate,
                              FLAG_old_gen_growth_time_ratio),
       gc_time_micros_(0),
       collections_(0) {
 }
 
 
@@ skipping 963 matching lines @@
     return 0;
   } else {
     ASSERT(total_time >= gc_time);
     int result = static_cast<int>((static_cast<double>(gc_time) /
                                    static_cast<double>(total_time)) * 100);
     return result;
   }
 }
 
 }  // namespace dart