Migrated test block 27 to Dart 2.0.

tests/corelib_2/stacktrace_fromstring_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 "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import "dart:async";
 
 void main() {
   StackTrace stack;
@@ skipping 25 matching lines @@
     c.stream.listen((v) {
       throw "Unexpected value: $v";
     }, onError: (e, s) {
       Expect.equals(string, s.toString());
       asyncEnd();
     });
     c.addError(0, stringTrace);
     c.close();
   });
 }