add Library and LanguageVersion classes, all declarations now have a library

Bug: https://github.com/dart-lang/language/issues/2839
Change-Id: Id09b2da070302b67f7c9921f1ad8a6d91a129f77
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311000
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>

https://dart.googlesource.com/sdk/+/b9bcfa70049abcd30eabad8b58855bea6dae1306