descending function Sorting data
Returns -1 if a is greater than b, or 1 if a is less than b, or 0.
This is the comparator function for reverse natural order.
Implementation
num descending(Object? a, Object? b) {
return a == null || b == null || a != a || b != b
? double.nan
: -(a as Comparable).compareTo(b);
}