“Rock music has made intermittent and sometimes dubious forays onto Broadway since Elvis and the Beatles met Ed Sullivan. And yet it has never become a staple sound — or even a frequent one — of musical theater, which has for the most part continued to descend from the early pioneers like Jerome Kern and George Gershwin, through Rodgers and Hammerstein and Hammerstein’s protégé Stephen Sondheim, to Mr. Sondheim’s many imitators or homage payers.”