[element model] migrate error_verifier (_enclosing{Class,Extension})

Change-Id: I36ffcb401da8448f1a1df7791af0a0974a0d0d75
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411082
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Auto-Submit: Phil Quitslund <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq