Fix UNUSED_ELEMENT_PARAMETER for unused field formal initializers.

[scheglov]

We would like to land https://dart-review.googlesource.com/c/sdk/+/226962 into Dart analyzer and fixing exiting violations.

List which issues are fixed by this PR. You must list at least one issue.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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.

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[guidezpl]

Tests can be fixed with flutter pub run grinder update-code-segments