- Do not repeatedly allocate and release Monitor objects.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index d7846a7f5a557ebf1c26020fedf5e0ed24abe9f8..8950958a85c12192cd229059c13bb24e7eae927d 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -31,6 +31,8 @@ Heap::Heap(Isolate* isolate,
            intptr_t max_old_gen_words,
            intptr_t max_external_words)
     : isolate_(isolate),
+      barrier_(new Monitor()),
+      barrier_done_(new Monitor()),
       new_space_(this, max_new_gen_semi_words, kNewObjectAlignmentOffset),
       old_space_(this, max_old_gen_words, max_external_words),
       read_only_(false),
@@ -48,6 +50,9 @@ Heap::Heap(Isolate* isolate,
 
 
 Heap::~Heap() {
+  delete barrier_;
+  delete barrier_done_;
+
   for (int sel = 0;
        sel < kNumWeakSelectors;
        sel++) {