Enables clean VM shutdown.

runtime/vm/message_handler.h

Index: runtime/vm/message_handler.h
diff --git a/runtime/vm/message_handler.h b/runtime/vm/message_handler.h
index 07fa5ab09b7a80c9240434f0b1f30a7cc11617c8..39cc8d7d43992a6ebbd49d35b8c54e94e87f56f1 100644
--- a/runtime/vm/message_handler.h
+++ b/runtime/vm/message_handler.h
@@ -36,8 +36,7 @@ class MessageHandler {
   // no longer has any live ports.  Abnormal termination occurs when
   // HandleMessage() indicates that an error has occurred during
   // message processing.
-  void Run(ThreadPool* pool,
-           StartCallback start_callback,
+  void Run(StartCallback start_callback,
            EndCallback end_callback,
            CallbackData data);
 
@@ -207,7 +206,7 @@ class MessageHandler {
   bool pause_on_exit_;
   bool paused_on_start_;
   bool paused_on_exit_;
-  ThreadPool* pool_;
+  bool running_;
   ThreadPool::Task* task_;
   StartCallback start_callback_;
   EndCallback end_callback_;