Add Future.timeout.

tests/lib/async/future_value_chain4_test.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.
 
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 
 class MyFuture implements Future {
   then(valueHandler, {onError}) {
     scheduleMicrotask(() { valueHandler(499); });
   }
   catchError(_, {test}) => null;
   whenComplete(_) => null;
   asStream() => null;
+  timeout(Duration timeLimit, [void onTimeout()]) => null;
 }
 
 main() {
   asyncStart();
   Completer completer = new Completer();
   completer.complete(new MyFuture());
   Expect.isTrue(completer.isCompleted);
   Expect.throws(() => completer.complete(42));
   completer.future.then((_) => asyncEnd());
 }