[annot-inf] internal error on unexpected widen_obj_type
[annot-inf] internal error on unexpected widen_obj_type

Summary:
Raise an internal error on a missing tvar in `widen_obj_type`. This codepath is likely dead,
I've never seen it get triggered.

Changelog: [internal]

Reviewed By: samwgoldman

Differential Revision: D32123997

fbshipit-source-id: 774435188ac6f1c556cd48223334a01840513f23

mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow