Dart: Futures -> Callbacks.

examples/dart/echo_client/lib/main.dart

 // Copyright 2015 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.
 
 import 'dart:async';
 
 import 'package:mojo/application.dart';
 import 'package:mojo/core.dart';
 import 'package:_mojo_for_test_only/mojo/examples/echo.mojom.dart';
 
 class EchoClientApplication extends Application {
   final EchoInterfaceRequest _echo = new EchoInterfaceRequest();
 
   EchoClientApplication.fromHandle(MojoHandle handle)
       : super.fromHandle(handle) {
     onError = ((_) {
       _closeHandles();
     });
   }
 
   @override
   void initialize(List<String> arguments, String url) {
     // See README.md for how to specify an alternate server on the command line.
     final server = (arguments.length > 0) ? arguments[1] : "dart_echo_server";
     connectToService(url.replaceAll("dart_echo_client", server), _echo);
 
-    _echo.echoString("hello world").then((response) {
-      print("${response.value}");
-    }).whenComplete(_closeHandles);
+    var c = new Completer();
+    _echo.echoString("hello world", (String response) {
+      print("${response}");
+      c.complete(null);
+    });
+    c.future.whenComplete(_closeHandles);
   }
 
   Future _closeHandles() async {
     await _echo.close();
     await close();
     MojoHandle.reportLeakedHandles();
   }
 }
 
 main(List args, Object handleToken) {
   MojoHandle appHandle = new MojoHandle(handleToken);
   new EchoClientApplication.fromHandle(appHandle);
 }