Migrate block 39 language/async_backwards ... /async_star_take.

tests/language_strong/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();
-  });
-}