[sdk] Mark List.empty as @Since("2.9")
[sdk] Mark List.empty as @Since("2.9")

2.9.0 is the first release that included List.empty:
https://api.dart.dev/stable/2.8.4/dart-core/List/List.empty.html (404)
https://api.dart.dev/stable/2.9.0/dart-core/List/List.empty.html (200)

Change-Id: Iddccfd9a38032da6353667fb032fcfcca26c2664
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/181803
Auto-Submit: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk