add method
Adds value
to the end of this list,
extending the length by one.
The list must be growable.
final numbers = <int>[1, 2, 3];
numbers.add(4);
print(numbers); // [1, 2, 3, 4]
Implementation
@override
void add(T? element) {
if (onAdd != null) onAdd!(element);
super.add(element);
}