Migrated test block 171 to Dart 2.0.

tests/lib/async/run_zoned6_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 'dart:async';
-import 'package:async_helper/async_helper.dart';
-
-main() {
-  asyncStart();
-  // Ensure that `runZoned`'s onError handles synchronous errors but delegates
-  // to the top-level when the handler returns false.
-  try {
-    runZoned(() {
-      throw 0;
-    }, onError: (e) {
-      Expect.equals(0, e);
-               if (false) //# 01: runtime error
-      asyncEnd();
-               throw e; //# 01: runtime error
-    });
-  } catch (e) {
-    // We should never see an error here.
-    if (false) //# 01: continued
-    rethrow;
-  }
-}