hsl method
Convert to HslColor.
Implementation
HslColor hsl() {
final num mx = max(max(red, green), blue),
mn = min(min(red, green), blue), ch = mx - mn;
final num light = (mx + mn) / (2 * 255);
final num sat = ch == 0 ? 0 :
ch / (light < 0.5 ? mx + mn : 2 * 255 - mx - mn);
final int lead = red > green ?
(red > blue ? 0 : 2) : (green > blue ? 1 : 2);
final num hue = 60 * (ch == 0 ? 0 :
lead == 0 ? ((green - blue) / ch) % 6 :
lead == 1 ? (blue - red) / ch + 2 : (red - green) / ch + 4);
return HslColor(hue, sat, light, alpha);
}