- Do not repeatedly allocate and release Monitor objects.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index 6fcc90cb85d67f4ca7601f1354bfb79b95220984..c9ac9faea5d67e5d3fcc59677bf83b27d13f4848 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -713,7 +713,9 @@ void GCMarker::MarkObjects(Isolate* isolate,
       // All marking done; detach code, etc.
       FinalizeResultsFrom(&mark);
     } else {
-      ThreadBarrier barrier(num_tasks + 1);
+      ThreadBarrier barrier(num_tasks + 1,
+                            heap_->barrier(),
+                            heap_->barrier_done());
       // Used to coordinate draining among tasks; all start out as 'busy'.
       uintptr_t num_busy = num_tasks;
       // Phase 1: Iterate over roots and drain marking stack in tasks.