@GwtIncompatible // NavigableMap
  private static class FilteredEntryNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractNavigableMap<K, V> {
    /*
     * It's less code to extend AbstractNavigableMap and forward the filtering logic to
     * FilteredEntryMap than to extend FilteredEntrySortedMap and reimplement all the NavigableMap
     * methods.
     */