Make Zone API strong mode clean.

tests/lib/async/zone_root_bind_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() {
   Expect.identical(Zone.ROOT, Zone.current);
   Zone forked = Zone.current.fork();
   var f = Zone.current.bindCallback(() {
     Expect.identical(Zone.ROOT, Zone.current);
-  }, runGuarded: false);
+  });
   forked.run(() {
     f();
   });
 }