sign property Null safety
Negative one, zero or positive 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;