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This is a necessary part of reworking nullability migration to stop using constraint variables entirely. Change-Id: If6c347c16882edfc2e8bdbeac761e09d8256c9bb Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/101622 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Commit-Queue: Paul Berry <paulberry@google.com>