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

runtime/vm/dart_api_state.cc

Index: runtime/vm/dart_api_state.cc
diff --git a/runtime/vm/dart_api_state.cc b/runtime/vm/dart_api_state.cc
index be30a2d5d5258a24423ce38af1ecc3ae4d4a7631..ed6ac3ed19e2671b5fc6fcd46e12ff721ce4da2c 100644
--- a/runtime/vm/dart_api_state.cc
+++ b/runtime/vm/dart_api_state.cc
@@ -19,9 +19,9 @@ BackgroundFinalizer::BackgroundFinalizer(Isolate* isolate,
     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();
 }
 
@@ -46,9 +46,9 @@ void BackgroundFinalizer::Run() {
   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();
   }
 }