then method
Combine comparators sequentially.
Creates a comparator which orders elements the same way as
this comparator, except that when two elements are considered
equal, the tieBreaker
comparator is used instead.
Implementation
Comparator<T> then(Comparator<T> tieBreaker) => (T a, T b) {
var result = this(a, b);
if (result == 0) result = tieBreaker(a, b);
return result;
};