ABC has announced they will air the “first look” at Marvel’s Agents of S.H.I.E.L.D. this Sunday night during the season finale of Once Upon a Time at 8|7c on ABC. After the show, you will be able to re-watch the promo on the official Agents of S.H.I.E.L.D. website at ABC.com where they will also “declassify” some information…