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>

samples/ffi/sqlite/lib/src/collections/closable_iterator.dart

@@ -7,7 +7,7 @@
 /// [ClosableIterator]s often use resources which should be freed after use.
 /// The consumer of the iterator can either manually [close] the iterator, or
 /// consume all elements on which the iterator will automatically be closed.
-abstract class ClosableIterator<T> extends Iterator<T> {
+abstract class ClosableIterator<T> implements Iterator<T> {
   /// Close this iterator.
   void close();