Frog shouldn't read libraries with --leg_only.

dart/frog/tests/leg_only/test_config.dart

 // Copyright (c) 2011, 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.
 
 #library("leg_only_test_config");
 
 #import("../../../tools/testing/dart/test_suite.dart");
 
 class LegOnlyTestSuite extends StandardTestSuite {
   LegOnlyTestSuite(Map configuration)
       : super(configuration,
               "leg_only",
               "frog/tests/leg_only/src",
               ["frog/tests/leg_only/leg_only.status"]);
-
-  List<String> additionalOptions(String filename) => ['--leg_only'];
 }