Background finalization.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 6131bf319bfc2d3fd048900c7c5bf52a75db3f6f..d13150ecdd30cd8667cf8a8ffee7f191b6a2caef 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -256,6 +256,10 @@ class Heap {
   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; }
+
   bool ShouldPretenure(intptr_t class_id) const;
 
   void SetupExternalPage(void* pointer, uword size, bool is_executable) {
@@ -353,6 +357,9 @@ class Heap {
   Monitor* barrier_;
   Monitor* barrier_done_;
 
+  Monitor* finalization_tasks_lock_;
+  intptr_t finalization_tasks_;
+
   // GC stats collection.
   GCStats stats_;