Add dart2native discontinuation to changelog
Add dart2native discontinuation to changelog

Follow-up to https://dart-review.googlesource.com/c/sdk/+/210241

Change-Id: I828adafb04ae92731378b2e372a127dc35de12cb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210921
Reviewed-by: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@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