withOpacity method
Returns a new color that matches this color with the alpha channel
replaced with the given opacity
(which ranges from 0.0 to 1.0).
Out of range values will have unexpected effects.
Implementation
@override
MacosColor withOpacity(double opacity) {
assert(opacity >= 0.0 && opacity <= 1.0);
return withAlpha((255.0 * opacity).round());
}