In honor of “Step Up All In,” a.k.a “Step Up 5,” opening this weekend (and not having any screenings for press; hmmm . . . .), here’s a delightful montage of 100 great movie dances, from 1921 (!) to 2010. You may note a few omissions (where’s “The Turning Point”? “Cabaret”?) or mistakes (“The Sound of Music” was not 1959, but six years later), but I just spent twelve minutes utterly mesmerized by this; you might, too.