Update CHANGELOG for 2.12.0.
Update CHANGELOG for 2.12.0.

- Reword null safety prose some.
- Mention other new language features.
- Clean up verbiage and formatting of other entries.

Change-Id: I9f444f4424cb4061ca70f9aa156f2d267fa5cda6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185620
Auto-Submit: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Reviewed-by: Vijay Menon <671ecbb18d31a1c038875027dcfe976af959ea9f@google.com>
Commit-Queue: Vijay Menon <671ecbb18d31a1c038875027dcfe976af959ea9f@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