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

runtime/vm/heap.h

 // 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.
 
 #ifndef VM_HEAP_H_
 #define VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 235 matching lines @@
   void PrintHeapMapToJSONStream(Isolate* isolate, JSONStream* stream) {
     old_space_.PrintHeapMapToJSONStream(isolate, stream);
   }
 #endif  // PRODUCT
 
   Isolate* isolate() const { return isolate_; }
 
   Monitor* barrier() const { return barrier_; }
   Monitor* barrier_done() const { return barrier_done_; }
 
-  Monitor* finalization_tasks_lock() const { return finalization_tasks_lock_; }
-  intptr_t finalization_tasks() const { return finalization_tasks_; }
-  void set_finalization_tasks(intptr_t count) { finalization_tasks_ = count; }
-
   void SetupExternalPage(void* pointer, uword size, bool is_executable) {
     old_space_.SetupExternalPage(pointer, size, is_executable);
   }
 
  private:
   class GCStats : public ValueObject {
    public:
     GCStats() {}
     intptr_t num_;
     Heap::Space space_;
@@ skipping 73 matching lines @@
   // The different spaces used for allocation.
   ALIGN8 Scavenger new_space_;
   PageSpace old_space_;
 
   WeakTable* new_weak_tables_[kNumWeakSelectors];
   WeakTable* old_weak_tables_[kNumWeakSelectors];
 
   Monitor* barrier_;
   Monitor* barrier_done_;
 
-  Monitor* finalization_tasks_lock_;
-  intptr_t finalization_tasks_;
-
   // GC stats collection.
   GCStats stats_;
 
   // This heap is in read-only mode: No allocation is allowed.
   bool read_only_;
 
   // GC on the heap is in progress.
   Monitor gc_in_progress_monitor_;
   bool gc_new_space_in_progress_;
   bool gc_old_space_in_progress_;
@@ skipping 32 matching lines @@
 // Note: During this scope, the code pages are non-executable.
 class WritableVMIsolateScope : StackResource {
  public:
   explicit WritableVMIsolateScope(Thread* thread);
   ~WritableVMIsolateScope();
 };
 
 }  // namespace dart
 
 #endif  // VM_HEAP_H_