[nnbd_migration] Don't visit spread elements twice
[nnbd_migration] Don't visit spread elements twice

This will be tested once TypeNameChecker is landed; visiting the
a TypeName in a spread twice triggers that assertion.

Change-Id: Ibd9ca8269a88205051c2fd6746d7e5d648dbd139
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133249
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@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