This reverts commit 4f8333e80e.
Reason for revert: causes breakages in google3
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>
Change-Id: I94ff95f72410a4e1ae80744971c4c920fecc1493
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287760
Reviewed-by: Martin Kustermann <kustermann@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Oleh Prypin <oprypin@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
