- Do not repeatedly allocate and release Monitor objects.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index cfc1b549d3e6d9b91e81db6ff94375ff741654bf..f116af55f56e99ad6689d0d882638ab9561809d9 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -244,6 +244,9 @@ class Heap {
 
   Isolate* isolate() const { return isolate_; }
 
+  Monitor* barrier() const { return barrier_; }
+  Monitor* barrier_done() const { return barrier_done_; }
+
   bool ShouldPretenure(intptr_t class_id) const;
 
   void SetupExternalPage(void* pointer, uword size, bool is_executable) {
@@ -329,6 +332,8 @@ class Heap {
   void GetMergedAddressRange(uword* start, uword* end) const;
 
   Isolate* isolate_;
+  Monitor* barrier_;
+  Monitor* barrier_done_;
 
   // The different spaces used for allocation.
   Scavenger new_space_;