Also removed a bunch of `CheckNotNullable`s that shouldn't be necessary any more. Any remaining non-sound-null-safety code runs today, and no more should be written. (And if it is, it'll mostly just err somewhere else, with a worse error message.)
Tested: New test added. Removed older tests checking for unsound null-safety.
Change-Id: I28626909cd8c1f91db6c61fc2b93042ed1b085dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/380780
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
Lasse R.H. Nielsen