This reverts commit 0c05e33836.
Reason for revert: breaks ~10% of G3 smoke suite.
Original change's description:
> Enforce current library restrictions.
>
> Mark all currently unimplementable types as `final`, or `sealed` for `num` and `final` for `Function`.
> Mark all current classes intended as mixins as `mixin class`.
>
> More additions and cleanup will follow,
> but this change should make everything keep working as today
> if we flip the switch.
>
> TEST= No new tests, very little actual change, covered by existing tests with few changes. Will add more tests when adding more modifiers.
>
> Change-Id: I40e724f823e7f88cdef186d2f73874df256e2f43
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/281683
> Reviewed-by: Siva Annamalai <asiva@google.com>
> Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
> Reviewed-by: Aske Simon Christensen <askesc@google.com>
> Reviewed-by: Slava Egorov <vegorov@google.com>
> Commit-Queue: Lasse Nielsen <lrn@google.com>
> Reviewed-by: Sigmund Cherem <sigmund@google.com>
> Reviewed-by: Johnni Winther <johnniwinther@google.com>
> Reviewed-by: Kallen Tu <kallentu@google.com>
# Not skipping CQ checks because original CL landed > 1 day ago.
Change-Id: Ib0cb5b7ec1a8c392bbf9bf4af8dc3efc0b27991d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/284187
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Ilya Yanok <yanok@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
029e0cec71