Description

This fixes outdated documentation in flutter_test.

Related issues

• Fixes [flutter_test] flutter_test_config testMain impossible in Dart 2.12.0 #70861.

Checklist

Before you create this PR, confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [Handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.