operator [] method
override
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 null value.
Methods like containsKey or putIfAbsent can be used if the distinction
is important.
Implementation
V operator [](Object key) => _map[key];