Migrated test block 171 to Dart 2.0.

tests/lib_2/async/stack_trace02_test.dart

 // Copyright (c) 2013, 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';
 import 'dart:async';
 
 main() {
   StackTrace trace;
   asyncStart();
   var f = new Future(() {
     throw "foo";
   });
   f.catchError((e, st) {
     Expect.equals("foo", e);
     Expect.isNotNull(st);
     asyncEnd();
   });
 }