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

tests/language_2/async_finally_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";
 
 foo() async {
   try {
     await 1;
     throw "error";
   } on String catch (e) {
     await 2;
     throw e;
   } finally {
     await 3;
   }
 }
 
 main() async {
   var error = "no error";
   try {
     await foo();
   } catch (e) {
     error = e;
   }
   Expect.equals("error", error);
 }