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

runtime/vm/heap.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/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 17 matching lines @@
 
 Heap::Heap(Isolate* isolate,
            intptr_t max_new_gen_semi_words,
            intptr_t max_old_gen_words,
            intptr_t max_external_words)
     : isolate_(isolate),
       new_space_(this, max_new_gen_semi_words, kNewObjectAlignmentOffset),
       old_space_(this, max_old_gen_words, max_external_words),
       barrier_(new Monitor()),
       barrier_done_(new Monitor()),
-      finalization_tasks_lock_(new Monitor()),
-      finalization_tasks_(0),
       read_only_(false),
       gc_new_space_in_progress_(false),
       gc_old_space_in_progress_(false) {
   for (int sel = 0;
        sel < kNumWeakSelectors;
        sel++) {
     new_weak_tables_[sel] = new WeakTable();
     old_weak_tables_[sel] = new WeakTable();
   }
   stats_.num_ = 0;
 }
 
 
 Heap::~Heap() {
   delete barrier_;
   delete barrier_done_;
-  delete finalization_tasks_lock_;
 
   for (int sel = 0;
        sel < kNumWeakSelectors;
        sel++) {
     delete new_weak_tables_[sel];
     delete old_weak_tables_[sel];
   }
 }
 
 
@@ skipping 807 matching lines @@
   Dart::vm_isolate()->heap()->WriteProtect(false);
 }
 
 
 WritableVMIsolateScope::~WritableVMIsolateScope() {
   ASSERT(Dart::vm_isolate()->heap()->UsedInWords(Heap::kNew) == 0);
   Dart::vm_isolate()->heap()->WriteProtect(true);
 }
 
 }  // namespace dart