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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 3c6c88b8049299086efe71ec19e2c6e858493fbd..af114b9decc6ef2639ab4f871f0c7a4a95f673fb 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1736,20 +1736,10 @@ void Isolate::Shutdown() {
   if (heap_ != NULL) {
     // Wait for any concurrent GC tasks to finish before shutting down.
     // TODO(koda): Support faster sweeper shutdown (e.g., after current page).
-    {
-      PageSpace* old_space = heap_->old_space();
-      MonitorLocker ml(old_space->tasks_lock());
-      while (old_space->tasks() > 0) {
-        ml.Wait();
-      }
-    }
-
-    // Wait for background finalization to finish before shutting down.
-    {
-      MonitorLocker ml(heap_->finalization_tasks_lock());
-      while (heap_->finalization_tasks() > 0) {
-        ml.Wait();
-      }
+    PageSpace* old_space = heap_->old_space();
+    MonitorLocker ml(old_space->tasks_lock());
+    while (old_space->tasks() > 0) {
+      ml.Wait();
     }
   }