This is a reland of commit f17e9732c5
Original change's description:
> [analyzer/cfe] Report an error when declaring a mixin class with final, interface, or sealed.
>
> Change-Id: Ia1393851dffaab55c31d04d4e81bfae83a7bd67f
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/283126
> Commit-Queue: Kallen Tu <kallentu@google.com>
> Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
> Reviewed-by: Johnni Winther <johnniwinther@google.com>
Change-Id: Ic8185f38477b2eeff624ee391868d7260e0ed4cd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/284486
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>