Use the async package's CancelableOperation.

lib/src/runner/browser/browser_manager.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.runner.browser.browser_manager;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:async/async.dart';
 import 'package:http_parser/http_parser.dart';
 import 'package:pool/pool.dart';
 
 import '../../backend/group.dart';
 import '../../backend/metadata.dart';
 import '../../backend/test.dart';
 import '../../backend/test_platform.dart';
-import '../../util/cancelable_future.dart';
 import '../../util/multi_channel.dart';
 import '../../util/remote_exception.dart';
 import '../../util/stack_trace_mapper.dart';
 import '../../utils.dart';
 import '../application_exception.dart';
 import '../environment.dart';
 import '../load_exception.dart';
 import '../runner_suite.dart';
 import 'browser.dart';
 import 'chrome.dart';
@@ skipping 246 matching lines @@
       Map test) {
     if (test == null) return null;
 
     var metadata = new Metadata.deserialize(test['metadata']);
     var testChannel = suiteChannel.virtualChannel(test['channel']);
     return new IframeTest(test['name'], metadata, testChannel,
         mapper: mapper);
   }
 
   /// An implementation of [Environment.displayPause].
-  CancelableFuture _displayPause() {
-    if (_pauseCompleter != null) return _pauseCompleter.future;
+  CancelableOperation _displayPause() {
+    if (_pauseCompleter != null) return _pauseCompleter.operation;
 
-    _pauseCompleter = new CancelableCompleter(() {
+    _pauseCompleter = new CancelableCompleter(onCancel: () {
       _channel.sink.add({"command": "resume"});
       _pauseCompleter = null;
     });
 
-    _channel.sink.add({"command": "displayPause"});
-    return _pauseCompleter.future.whenComplete(() {
+    _pauseCompleter.operation.value.whenComplete(() {
       _pauseCompleter = null;
     });
+
+    _channel.sink.add({"command": "displayPause"});
+
+    return _pauseCompleter.operation;
   }
 
   /// The callback for handling messages received from the host page.
   void _onMessage(Map message) {
     assert(message["command"] == "resume");
     if (_pauseCompleter == null) return;
     _pauseCompleter.complete();
   }
 
   /// Closes the manager and releases any resources it owns, including closing
@@ skipping 13 matching lines @@
 class _BrowserEnvironment implements Environment {
   final BrowserManager _manager;
 
   final Uri observatoryUrl;
 
   final Uri remoteDebuggerUrl;
 
   _BrowserEnvironment(this._manager, this.observatoryUrl,
       this.remoteDebuggerUrl);
 
-  CancelableFuture displayPause() => _manager._displayPause();
+  CancelableOperation displayPause() => _manager._displayPause();
 }