[vm] Add class modifiers to dart:isolate

SendPort/ReceivePort/RawReceivePort become interface classes as there
are existing implementations of those classes (e.g. `package:isolate`
has `Multiplex*Port` classes those).

Since Capability is implemented by SendPort it also becomes an interface
class.

All remaining classes become final - we can always be less restrictive
in the future if needed.

It seems good practice to also add modifiers to private classes (one
reason is that private classes can be exported via e.g. typdef).

TEST=ci

CoreLibraryReviewExempt: No API changes except for modifiers, vm-only library.
Change-Id: I5edf04624a5dc588ac5f49bb898f29b5bc121d5f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288201
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>