The Strokes brought out surprise guest Charli XCX to perform “Take It or Leave It” during the band’s headlining set Saturday at the Outside Lands festival. “We got a special little guest. A big guest. A superstar,” Julian Casablancas told the crowd before alluding to the fact that both artists released new albums — the Strokes’ Reality Awaits, Charli’s Music, Fashion, Film — on the same day in July. “We had a great battle. Congratulations, you defeated us in the album sales. You’re new queen of everything, Charli XCX.” The group and the singer — who headlined the previous night at the San Francisco fest — then launched into the Is This It? closer: