Delete NativeMessageHandler once a native port closes.

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 c52431d03ddec75ed072bcb9deb947e2f35b3385..825e3d38ad858e08dd7831445fb46db9e419435e 100644
--- a/runtime/vm/message_handler.h
+++ b/runtime/vm/message_handler.h
@@ -187,6 +187,10 @@ class MessageHandler {
   // This is used to delete handlers when their last live port is closed.
   virtual bool OwnedByPortMap() const { return false; }
 
+  // Requests deletion of this message handler when the next task
+  // completes.
+  void RequestDeletion();
+
   void increment_live_ports();
   void decrement_live_ports();
   // ------------ END PortMap API ------------
@@ -241,6 +245,7 @@ class MessageHandler {
   bool should_pause_on_exit_;
   bool is_paused_on_start_;
   bool is_paused_on_exit_;
+  bool delete_me_;
   int64_t paused_timestamp_;
   ThreadPool* pool_;
   ThreadPool::Task* task_;