Though Stephen Sondheim is considered a master of musical theatre, the esteem isn’t shared by a popular audience. So should his pieces be performed by opera companies instead? “Sondheim himself makes no secret of thinking otherwise. He even affects not to like opera, and has never written a work intended for opera-house production. Still, the alacrity with which ‘Sweeney Todd’ and ‘A Little Night Music’ have been taken up by major opera companies in America and elsewhere raises the question of whether they might really have been, all along, modern operas in disguise.”