sign property Null safety
Returns minus one, zero or plus one depending on the sign and numerical value of the number.
Returns minus one if the number is less than zero, plus one if the number is greater than zero, and zero if the number is equal to zero.
Returns NaN if the number is the double NaN value.
Returns a number of the same type as this number.
-0.0.sign == -0.0.
The result satisfies:
n == n.sign * n.abs()
for all numbers
n (except NaN, because NaN isn't
== to itself).
num? get sign => value?.sign;