Cory Stearns, the longtime American Ballet Theater star, had planned to dance his final performance with the company on June 27. “It wouldn’t have been my first choice,” Stearns, 40, said in an interview. “It would have been nice to dance a few more years.” His farewell was supposed to be in the title role of “Onegin,” which he has performed 10 times, by his count, since he joined the company 21 years ago. “It was always a dream of mine to retire with ‘Manon,’” he said.