Returns a new Duration representing this Duration negated.
The returned Duration has the same length as this one, but will have the
opposite sign of this one.
Source
/**
* Returns a new `Duration` representing this `Duration` negated.
*
* The returned `Duration` has the same length as this one, but will have the
* opposite sign of this one.
*/
Duration operator -() => new Duration._microseconds(-_duration);