Migrate block 163 to Dart 2.0.

tests/language_2/vm/async_await_catch_stacktrace_test.dart

 // Copyright (c) 2017, 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:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 void main() {
   x() async {
     print("Starting!");
@@ skipping 36 matching lines @@
     print("Ending!");
   }
 
   asyncStart();
   x().then((_) => asyncEnd());
 }
 
 runAsync() async {
   throw 'oh no!';
 }