This PR is the 9ᵗʰ step in the journey to solve issue #136139 and make the entire Flutter repo more readable.

(previous pull requests: #139048, #139882, #141591, #142279, #142634, #142793, #143293, #143496)

I did a pass through all of `packages/flutter/lib/src/` and found an abundance of `switch` statements to improve. Whereas #143496 focused on in-depth refactoring, this PR is full of simple, straightforward changes. (I ended up making some more complicated changes in `rendering/` and will file those separately after this PR is done.)