operator  method Null safety
- Object? key
The value for the given
key is not in the map.
Some maps allow
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 being there with a
Methods like containsKey or putIfAbsent can be used if the distinction
@override EndpointChain? operator (Object? key) => _map[key];