And disable implicit downcasts in the nnbd_migration package.
Change-Id: I00c95d709215a12c04e71ee5da1fb9c4af7b5d3c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/111266
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>