Migrate language/async_backwards... ... language/async_star_take...

tests/language_2/async_star_stream_take_test.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.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 Stream makeStream(int n) async* {
   for (int i = 0; i < n; i++) yield i;
 }
 
 main() {
   f(Stream s) async {
     var r = 0;
     await for (var v in s.take(5)) r += v;
     return r;
   }
 
   asyncStart();
   f(makeStream(10)).then((v) {
     Expect.equals(10, v);
     asyncEnd();
   });
 }