This PR is step 12 in the journey to solve issue #136139 and make the
entire Flutter repo more readable.
Most of it involves implementing switch expressions, and there's also a
few other random things that I wanted to clean up a bit.
Fixes https://github.com/flutter/flutter/issues/143482
This brings in the gallery more or less as is:
* Removed localizations
* Ensure tests still run (locally verified, will switch CI later).
* Removed deferred components
* Fixup pubspec