*<Null safety>*
V update (K key, V update(V), {V ifAbsent()}) override
@override V update(K key, V Function(V) update, {V Function()? ifAbsent}) => _base.update(_canonicalize(key), (pair) { var value = pair.value; var newValue = update(value); if (identical(newValue, value)) return pair; return MapEntry(key, newValue); }, ifAbsent: ifAbsent == null ? null : () => MapEntry(key, ifAbsent())).value;