Make use of Completer.isComplete in pub and pkg/scheduled_test.

pkg/scheduled_test/test/utils.dart

 // Copyright (c) 2013, 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_utils;
 
 import 'dart:async';
 
 import 'package:scheduled_test/src/utils.dart';
 import 'package:scheduled_test/src/mock_clock.dart' as mock_clock;
 
 export 'package:scheduled_test/src/utils.dart';
 
 /// Wraps [input] to provide a timeout. If [input] completes before
 /// [milliseconds] have passed, then the return value completes in the same way.
 /// However, if [milliseconds] pass before [input] has completed, [onTimeout] is
 /// run and its result is passed to [input] (with chaining, if it returns a
 /// [Future]).
 ///
 /// Note that timing out will not cancel the asynchronous operation behind
 /// [input].
 Future timeout(Future input, int milliseconds, onTimeout()) {
-  bool completed = false;
   var completer = new Completer();
   var timer = new Timer(new Duration(milliseconds: milliseconds), () {
-    completed = true;
-    chainToCompleter(new Future.immediate(null).then((_) => onTimeout()),
-        completer);
+    chainToCompleter(new Future.of(onTimeout), completer);
   });
   input.then((value) {
-    if (completed) return;
+    if (completer.isCompleted) return;
     timer.cancel();
     completer.complete(value);
   }).catchError((e) {
-    if (completed) return;
+    if (completer.isCompleted) return;
     timer.cancel();
     completer.completeError(e.error, e.stackTrace);
   });
   return completer.future;
 }
 
 /// Returns a [Future] that will complete in [milliseconds].
 Future sleep(int milliseconds) {
   var completer = new Completer();
   mock_clock.newTimer(new Duration(milliseconds: milliseconds), () {
     completer.complete();
   });
   return completer.future;
 }