SassColor.hsl constructor
Creates a color in ColorSpace.hsl.
If null is passed for alpha, that indicates that it's a missing
component. In most cases, this is equivalent to the color being
transparent.
Throws a RangeError if alpha isn't between 0 and 1.
Implementation
factory SassColor.hsl(num? hue, num? saturation, num? lightness,
[num? alpha = 1]) =>
SassColor.forSpaceInternal(ColorSpace.hsl, hue?.toDouble(),
saturation?.toDouble(), lightness?.toDouble(), alpha?.toDouble());