~~Using the same priority order as a Border without borderRadius, it is possible to draw them on top of each other. This is better than the current behavior (crash!) and would work well for a "one color on top, another on bottom" scenario.~~

~~With this, if approved, we move the current number of possible exceptions from 4 to 1 (`BoxShape.circle` + `borderRadius`).~~

~~It is kind of odd how `borderRadius.zero` to `borderRadius != BorderRadius.zero` change, but I think it is better than crashing. Alternatively, we just remove the "original function" and see if any goldens are affected.~~

<img width="448" alt="image" src="https://user-images.githubusercontent.com/351125/236550350-7499d758-5b44-40e6-9105-32671eb21998.png">

Another one for @gspencergoog. If this works, we could make the paint method public and re-use in the InputBorder PR (if that's also approved). Single line fix.