This PR is the 8ᵗʰ 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)

I did a pass through all of `packages/flutter/lib/src/` and found a whole bunch of `switch` statements to improve: most of them were really simple, but many involved some thorough refactoring.

This pull request is just the complicated stuff. 😎 I'll make comments to describe the changes, and then in the future there will be another PR (and it'll be much easier to review than this one).