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 <[email protected]>
Reviewed-by: Vijay Menon <[email protected]>
Commit-Queue: Vijay Menon <[email protected]>

Author: munificent