Tidy some tests after stable Dart release

[srawlins]

analyzer 6.8.0 is the version I get with dart pub get using Dart 3.5.0.

The rest of the tidying is removing old experiments.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[srawlins]

CC @dart-lang/analyzer-team