Returns the value for the given key or null if key is not in the map.
Some maps allows keys to have null as a value,
For those maps, a lookup using this operator does cannot be used to
distinguish between a key not being in the map, and the key having a null
value.
Methods like containsKey or putIfAbsent can be use if the distinction
is important.
Source
operator [](Object key) {
_ensureIsntReservedKey('$key');
return _values[key];
}