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

tests/language_2/async_rethrow_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";
 
 const String exceptionString = "exceptionalString";
 
 throwString() async {
@@ skipping 29 matching lines @@
     Expect.fail("Exception not thrown");
   }, onError: (e) {
     Expect.equals(exceptionString, e);
   });
 }
 
 main() {
   testThrow();
   testRethrow();
 }