Revert "Rewrite Futures."

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}) {
-    runAsync(() { valueHandler(499); });
-  }
-  catchError(_) => null;
-  whenComplete(_) => null;
-}
-
-main() {
-  asyncStart();
-  Completer completer = new Completer();
-  completer.complete(new MyFuture());
-  Expect.isTrue(completer.isCompleted);
-  Expect.throws(() => completer.complete(42));
-  completer.future.then((_) => asyncEnd());
-}