[Dart] Method overriding

void main() {
	TimesTwo tt = TimesTwo(2);
	print(tt.calculate());
	
	TimesFour tf = TimesFour(2);
	print(tf.calculate());
}

class TimesTwo {
	final int number;

	TimesTwo(
		this.number,
	);

	int calculate() {
		return number * 2;
	}
}

class TimesFour extends TimesTwo {
	TimesFour(int number,) : super(number);

	@override
	int calculate() {
		return super.number * 4
	}
}