operator  method Null safety
- Object? key
Returns the value for the given
key or null if
key is not in the map.
Some maps allow keys to have
null as a value.
For those maps, a lookup using this operator cannot distinguish between a
key not being in the map and the key having a
Methods like containsKey or putIfAbsent can be used if the distinction
V? operator (Object? key) => _map[key];