code: @Override
    @GwtIncompatible("MapMakerInternalMap")
    <K, V> MapMakerInternalMap<K, V> makeCustomMap() {
      return new MapMakerInternalMap<K, V>(this);
    }
docs: Returns a MapMakerInternalMap for the benefit of internal callers that use features of
  that class not exposed through ConcurrentMap.