“Avengers: Infinity War” (coming May 4) “So this is it — it’ all been leading to this,” is the first line of the this 30-second spot for Marvel’s sequel. Yet even with the Avengers, including Tony Stark (Robert Downey Jr.) a.k.a. Iron Man, colliding with the Guardians of the Galaxy, the ad seems underwhelming compared with the exhilarating trailer for the imminent “Black Panther.” Don’t blink or you’ll miss that film’s star, Chadwick Boseman, here. Fans may be the ones asking, “So this is it?”