fix: joining if-cases

[tenhobi]

Fixes "Join if statement" assist for if-cases. Fixes #56715

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https://dart-review.googlesource.com/c/sdk/+/390632 has been updated with the latest commits from this pull request.

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https://dart-review.googlesource.com/c/sdk/+/390632 has been updated with the latest commits from this pull request.