Handle FieldDeclaration in ConvertToFunctionDeclaration

[AgamAgarwal]

Fixes #61815

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[FMorschel]

Thanks for fixing this!

[FMorschel]

Note that you'll need to close the issue manually after that CL lands. If you can ask a reviewer to change the description to contain Fixes: <issue-url>, it may close the issue. Otherwise, it will, as stated there, close this PR and not merge it. The merging happens with Copybara somewhere else. And since this will not be merged, it doesn't close the linked issue.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/457460 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/457460 has been updated with the latest commits from this pull request.