Update Dart version, migrate deprecations, resolve formatting blocker

[Piinks]

In another PR, the formatter was complaining in CI.
This change updates the packages to the latest version of Dart and fixes the formatting issue.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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

[Piinks]

This appears to be more complicated than I thought. Local formatting yields a different result than CI.
Will close for now while I investigate further, the new formatter is likely getting mixed up in here somewhere.

[Piinks]

I think I found it

[auto-submit]

auto label is removed for flutter/assets-for-api-docs/255, due to - The status or check suite Test Assets for APIs has failed. Please fix the issues identified (or deflake) before re-applying this label.

[Piinks]

Thanks @domesticmouse!

[goderbauer]

Local formatting yields a different result than CI.

For future archeologists, the reason for this buggy behavior is dart-lang/dart_style#1617