Remove generated directories with 100s of files.

client/dom/generated/src/frog/_HTMLOptionElementFactoryProvider.dart

-// Copyright (c) 2012, 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.
-
-class _HTMLOptionElementFactoryProvider {
-  factory HTMLOptionElement([String data = null, String value = null,
-                             bool defaultSelected = null, bool selected = null])
-      native
-'''
-if (data == null) return new Option();
-if (value == null) return new Option(data);
-if (defaultSelected == null) return new Option(data, value);
-if (selected == null) return new Option(data, value, defaultSelected);
-return new Option(data, value, defaultSelected, selected);
-''';
-}