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
Color withOpacity(double opacity) {
assert(opacity >= 0.0 && opacity <= 1.0, 'opacity must be a double between (inclusive) 0.0 and 1.0');
return withAlpha((255.0 * opacity).round());
}