re-enable dartrfb

[Goddchen]

since the deprecated lint rules have been removed

See these 2 PRs: Goddchen/dart-rfb#26 and Goddchen/dart-rfb#25

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• 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.

[auto-submit]

auto label is removed for flutter/tests/427, due to This PR has not met approval requirements for merging. The PR author is not a member of flutter-hackers and needs 1 more review(s) in order to merge this PR.

• Merge guidelines: A PR needs at least one approved review if the author is already part of flutter-hackers or two member reviews if the author is not a flutter-hacker before re-applying the autosubmit label. Reviewers: If you left a comment approving, please use the "approve" review action instead.

[srawlins]

Thanks!