Make adding a pending deopt atomic.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index a90193af05e31f99a3d70aa5d20becd7420382de..bb2efbfb1643d5e73174e12cccf019b326e2b56e 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -404,7 +404,10 @@ class Isolate : public BaseIsolate {
     return object_id_ring_;
   }
 
-  MallocGrowableArray<PendingLazyDeopt>* pending_deopts() {
+  void AddPendingDeopt(uword fp, uword pc);
+  uword FindPendingDeopt(uword fp) const;
+  void ClearPendingDeoptsAtOrBelow(uword fp) const;

[siva, 2016/10/10 04:10:26]

This function is marked as const but it seems to modify pending_deopts_ how come
the compiler is not complaining.

+  MallocGrowableArray<PendingLazyDeopt>* pending_deopts() const {
     return pending_deopts_;
   }
   bool IsDeoptimizing() const { return deopt_context_ != NULL; }