This PR is the 7ᵗʰ 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)

This pull request covers everything in `packages/flutter/lib/src/widgets/`. Most of it should be really straightforward, but there was some refactoring in the `getOffsetToReveal()` function in `two_dimensional_viewport.dart`. I'll add some comments to describe those changes.