Adjusted language/assign/to_type_test.dart to avoid irrelevant nnbd type error.
Adjusted language/assign/to_type_test.dart to avoid irrelevant
nnbd type error.

By the way, I don't understand why the trybots didn't complain about
this, all trybots were green when
https://dart-review.googlesource.com/c/sdk/+/134224 was auto-submitted.

Change-Id: Id410fc18e9ebf9be6c180740067857e2309f47ef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134291
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Commit-Queue: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Auto-Submit: Erik Ernst <251bb01623bfda2cd9151e339a6e783ca432874c@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