Dart: Wait to handle events on a Stub until it makes sense to do it.

mojo/dart/apptests/test_apps/interfaces/pingpong_service.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [DartPackage="_mojo_for_test_only"]
 module test;
 
 [ServiceName="test::PingPongService"]
 interface PingPongService {
   // Register the client interface with the service.
   SetClient(PingPongClient client);
 
   // Typically calls PingPongClient::Pong(ping_value + 1)
   Ping(uint16 ping_value);
 
   // Ping the PingPongService at |url| |count| times and return true
   // after the final Pong() is received.
   PingTargetURL(string url, uint16 count) => (bool ok);
 
   // Ping the PingPongService |count| times and return true
   // after the final Pong() is received.
   PingTargetService(PingPongService service, uint16 count) => (bool ok);
 
   // Return a PingPongService
   GetPingPongService(PingPongService& service);
 
+  // Return a PingPongService after some delay
+  GetPingPongServiceDelayed(PingPongService& service);
+
   // Make it stop.
   Quit();
 };
 
 interface PingPongClient {
   Pong(uint16 pong_value);
 };