Treat background finalization as another GC task so it won't run in parallel with the marker.

runtime/vm/dart_api_state.cc

 // Copyright (c) 2016, 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/dart_api_state.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
 #include "vm/thread.h"
 #include "vm/timeline.h"
 
 namespace dart {
 
 BackgroundFinalizer::BackgroundFinalizer(Isolate* isolate,
                                          FinalizationQueue* queue) :
     isolate_(isolate),
     queue_(queue) {
   ASSERT(FLAG_background_finalization);
-  MonitorLocker ml(isolate->heap()->finalization_tasks_lock());
-  isolate->heap()->set_finalization_tasks(
-      isolate->heap()->finalization_tasks() + 1);
+  PageSpace* old_space = isolate->heap()->old_space();
+  MonitorLocker ml(old_space->tasks_lock());
+  old_space->set_tasks(old_space->tasks() + 1);
   ml.Notify();
 }
 
 
 void BackgroundFinalizer::Run() {
   bool result = Thread::EnterIsolateAsHelper(isolate_,
                                              Thread::kFinalizerTask);
   ASSERT(result);
 
   {
     Thread* thread = Thread::Current();
     TIMELINE_FUNCTION_GC_DURATION(thread, "BackgroundFinalization");
     TransitionVMToNative transition(thread);
     for (intptr_t i = 0; i < queue_->length(); i++) {
       FinalizablePersistentHandle* handle = (*queue_)[i];
       FinalizablePersistentHandle::Finalize(isolate_, handle);
     }
     delete queue_;
   }
 
   // Exit isolate cleanly *before* notifying it, to avoid shutdown race.
   Thread::ExitIsolateAsHelper();
 
   {
-    Heap* heap = isolate_->heap();
-    MonitorLocker ml(heap->finalization_tasks_lock());
-    heap->set_finalization_tasks(heap->finalization_tasks() - 1);
+    PageSpace* old_space = isolate_->heap()->old_space();
+    MonitorLocker ml(old_space->tasks_lock());
+    old_space->set_tasks(old_space->tasks() - 1);
     ml.Notify();
   }
 }
 
 }  // namespace dart