* Copyright (C) 2012 The Guava Authors
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
GWT can serialize an absent
Optional on its own, but the resulting object is a
different instance than the singleton
Absent.INSTANCE, which breaks equality. We
implement a custom serializer to maintain the singleton property.