Fix invalid downcast in pkg/nnbd_migration

Change-Id: Ie2a61f08a57ed741939c63a9ac7e07cf7f19c74f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144780
Commit-Queue: Paul Berry <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: stereotype441