operator [] method
The value for the given key, or null if 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 null value.
Methods like containsKey or putIfAbsent can be used if the distinction
is important.
Implementation
@override
dynamic operator [](Object? key) {
  if (key is! String) {
    if (key is int) {
      return _data[key];
    }
    return null;
  }
  final index = _result._calculatedIndexes[key];
  if (index == null) return null;
  return columnAt(index);
}