This pull request is part of the effort to solve issue #136139.

The previous [`switch` expressions PR](https://github.com/flutter/flutter/pull/143496) was comprised of many simple changes throughout `flutter/lib/src/`, but due to some more in-depth refactoring in `flutter/lib/src/rendering/`, I decided to submit the changes to this directory as a separate pull request.

There was really just one function that I changed significantly; I'll add a comment for explanation.