Migrated test block 168 to Dart 2.0.

tests/lib_2/async/dart2js_uncaught_error_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 "dart:async";
 import "dart:js";
 import "dart:collection" show Queue;
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
@@ skipping 27 matching lines @@
     }
   }
 
   () async {
     () async {
       throwit();
     }();
     throwit();
   }();
 }