我在Flutter网站上阅读了特定于平台的插件/渠道的介绍，并浏览了一些简单的插件示例，例如url_launcher：

// Copyright 2017 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:flutter/services.dart';

const _channel = const MethodChannel('plugins.flutter.io/url_launcher');

/// Parses the specified URL string and delegates handling of it to the
/// underlying platform.
///
/// The returned future completes with a [PlatformException] on invalid URLs and
/// schemes which cannot be handled, that is when [canLaunch] would complete
/// with false.
Future<Null> launch(String urlString) {
  return _channel.invokeMethod(
    'launch',
    urlString,
  );
}

在窗口小部件测试或集成测试中，如何模拟或存根通道，这样我不必依靠实际设备（运行Android或iOS）说实际上启动URL？