@SuppressWarnings("unchecked")
  static final Comparator<?> NATURAL_ORDER = Ordering.natural();

  // This reference is only used by GWT compiler to infer the keys and values
  // of the map that needs to be serialized.
  private @Nullable Comparator<? super K> unusedComparatorForSerialization;
  private @Nullable K unusedKeyForSerialization;
  private @Nullable V unusedValueForSerialization;