This reverts commit e293eb809d.
Reason for revert: Breaks G3.
Original change's description:
> Reland "Add more `interface` and `final` modifiers to `dart:core`."
>
> This is a reland of commit 4f8333e80e
>
> Blocked on Flutter patch: https://github.com/flutter/engine/pull/40175
>
> Original change's description:
> > Add more `interface` and `final` modifiers to `dart:core`.
> >
> > Make intent explicit for classes which are intended as interfaces,
> > or which are not intended to be subclassed.
> >
> > Mainly classes which are pure interfaces are marked as such,
> > and platform-specific classes not intended for subclassing
> > are made `final`.
> >
> > The `final` classes includes `BigInt`, which is written to assume
> > that arguments inherit its private members
> > (it runs `_ensureSystemBigInt` on arguments).
> >
> > It also includes the `Expando`, `WeakReference` and `Finalizer` classes,
> > which are just intended as stand-alone implementation classes for accessing
> > platform-specific functionality.
> >
> > Change-Id: Ib770c265edff127a289a67fe72d15b9ff0499407
> > Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287180
> > Reviewed-by: Stephen Adams <sra@google.com>
> > Commit-Queue: Lasse Nielsen <lrn@google.com>
> > Reviewed-by: Aske Simon Christensen <askesc@google.com>
> > Reviewed-by: Nate Bosch <nbosch@google.com>
> > Reviewed-by: Martin Kustermann <kustermann@google.com>
>
> CoreLibraryReviewExempt: Accepted in original CL.
> Change-Id: Id713edede4228801bb097a64734be4f2187f51a9
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288206
> Reviewed-by: Aske Simon Christensen <askesc@google.com>
> Commit-Queue: Lasse Nielsen <lrn@google.com>
> Reviewed-by: Nate Bosch <nbosch@google.com>
> Reviewed-by: Martin Kustermann <kustermann@google.com>
Change-Id: I05ff2fe44e5a13f20725eb122963d20243062c7a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288503
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Ilya Yanok <yanok@google.com>
936cdeeb2f