Enable dartanalyze warnings.

services/dart/test/echo/main.dart

 // 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.
 
 import 'dart:async';
 import 'mojo:application';
 import 'mojo:bindings';
 import 'mojo:core';
 
 import 'package:services/dart/test/echo_service.mojom.dart';
 
 class EchoServiceImpl implements EchoService {
   EchoServiceStub _stub;
   Application _application;
 
   EchoServiceImpl(this._application, MojoMessagePipeEndpoint endpoint) {
     _stub = new EchoServiceStub.fromEndpoint(endpoint, this);
   }
 
-  echoString(String value, Function responseFactory) {
+  echoString(String value, [Function responseFactory = null]) {
     if (value == "quit") {
       _stub.close();
     }
     return new Future.value(responseFactory(value));
   }
 }
 
 class EchoApplication extends Application {
   EchoApplication.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   @override
   void acceptConnection(String requestorUrl, String resolvedUrl,
       ApplicationConnection connection) {
     connection.provideService(
         EchoServiceName, (endpoint) => new EchoServiceImpl(this, endpoint));
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   String url = args[1];
   new EchoApplication.fromHandle(appHandle);
 }