Constructing colors using `fromRGBO` should return the same values as the CSS
`rgba()` notation. rgba(0, 0, 255, 0.5) is the same as `#0000ff80`
However `fromRGBO` sometimes creates a color with an off-by-one alpha value:
expect(Color.fromRGBO(0, 0, 255, 0.5), Color(0x800000ff));
Expected: Color:<Color(0x800000ff)>
Actual: Color:<Color(0x7f0000ff)>
If we use `withOpacity` to create the same color, it returns the correct color:
expect(Color.fromRGBO(0, 0, 255, 1).withOpacity(0.5), Color(0x800000ff));
This should also be changed in lib/web_ui/lib/src/ui/painting.dart in a
followup change.
9f2daad648