Anyone not convinced by Wild Flag’s eponymous debut album may be hastily revisiting that view after witnessing this performance of “Romance” on David Letterman’s show last night. The band has carefully refined its aesthetic through bouts of touring, and here the angular verses and chant-along chorus slot together with an ease that makes it sound like this group has been together forever. There’s some neat riffage exchange between Carrie Brownstein and Mary Timony, who seem to love playing off each other as the duel frontwomen of the band. Even Letterman seems impressed, in his understated way. “Pretty good,” he claims, at the close of the performance.