Reland "Add more interface and final modifiers to dart:core."

This is another reland of 4f8333e80e.
Third time is the charm.

CoreLibraryReviewExempt: Reland of accepted CL.
Change-Id: I4ea8326af91c168b044d252162571d3fe697e4b0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289826
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Reviewed-by: Ben Konyi <bkonyi@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();