invalid method
Returns true if the date
is invalid for this DateValidator.
This is the opposite of valid. Implementations that return true for invalid should also return false for valid.
Usually, this will be implemented as !valid(date)
by
DateValidatorMixin. However, if there is a simpler way to check
for invalid dates, it can be implemented here.
Implementation
@override
bool invalid(DateTime date) => !valid(date);