Background finalization.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 2c92b17c52f3692024ff02d1c4f8fd54507c8123..46fd7617790766db693684c443e40bc891616be5 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -35,6 +35,8 @@ Heap::Heap(Isolate* isolate,
       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),
@@ -52,6 +54,7 @@ Heap::Heap(Isolate* isolate,
 Heap::~Heap() {
   delete barrier_;
   delete barrier_done_;
+  delete finalization_tasks_lock_;
 
   for (int sel = 0;
        sel < kNumWeakSelectors;