Update for Flutter 3.22

[domesticmouse]

Update codelabs. Mixture of dart fix and re-running the codelab rebuild scripts, with random required fixups.

Pre-launch Checklist

• [x] I read the Effective Dart: Style recently, and have followed its advice.
• [x] I signed the CLA.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[domesticmouse]

FYI @filiph