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As a preparatory step for re-landing https://dart-review.googlesource.com/c/sdk/+/91170 and making these two restored problems to warnings. R=brianwilkerson@google.com, paulberry@google.com Change-Id: I6266632ade84743b5725c54475fe402902576c00 Reviewed-on: https://dart-review.googlesource.com/c/91420 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Commit-Queue: Konstantin Shcheglov <scheglov@google.com>