Creates a color swatch with a variety of shades.
primary argument should be the 32 bit ARGB value of one of the
values in the swatch, as would be passed to the new Color constructor
for that same color, and as is exposed by value. (This is distinct from
the specific index of the color in the swatch.)
const MaterialColor(int primary, Map<int, Color> swatch) : super(primary, swatch);