The bar for gala performances marking milestones at the Music Center got set pretty high when it opened in December 1964. Jascha Heifetz, Van Cliburn, Frank Sinatra, Diahann Carroll, Vikki Carr and Tony Bennett were among the luminaries in an opening-week roster spanning three evenings. The lineup for the Music Center’s 50th anniversary gala on Dec. 6 at the Dorothy Chandler Pavilion has been fleshed out with that tradition in mind, but there’s also a trump card to be played that wasn’t available at the beginning -- 50 years of performance history on Music Center stages. The names announced Monday include Phylicia Rashad, Edward James Olmos, playwright David Henry...