Don't allow oob message interrupts while executing constant expressions in the parser.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 77b1fe690e81e508174911a8b028a1208007300d..9bca13488f2b7c753898a81c948fb041e4871386 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -286,6 +286,9 @@ class Isolate : public BaseIsolate {
   RawError* HandleInterrupts();
   uword GetAndClearInterrupts();
 
+  void DeferMessageInterrupts();
+  void RestoreMessageInterrupts();
+
   // Marks all libraries as loaded.
   void DoneLoading();
 
@@ -744,6 +747,8 @@ class Isolate : public BaseIsolate {
   Simulator* simulator_;
   Mutex* mutex_;  // protects stack_limit_, saved_stack_limit_, compiler stats.
   uword saved_stack_limit_;
+  uword deferred_interrupts_mask_;
+  uword deferred_interrupts_;
   uword stack_overflow_flags_;
   int32_t stack_overflow_count_;
   MessageHandler* message_handler_;
@@ -864,6 +869,7 @@ REUSABLE_HANDLE_LIST(REUSABLE_FRIEND_DECLARATION)
   friend class ServiceIsolate;
   friend class Thread;
   friend class Timeline;
+  friend class IsolateTestHelper;
 
   DISALLOW_COPY_AND_ASSIGN(Isolate);
 };