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

(previous pull requests: #139048, #139882, #141591)

This one is covering files in `packages/flutter/lib/src/foundation/` and `packages/flutter/lib/src/material/`.  
The `material/` directory is pretty big though, so for now I just did the files that start with `a`, `b`, and `c`.