MassOpera heads to City Winery on May 10 to present “The Mirror Cabaret.” The showcase will feature a showdown between opera classics and their musical theater counterparts. To that effect, audiences will hear back-to-back showcases from such works as “Aida” (and “Aida”); “Rusalka” and “The Little Mermaid;” “Madama Butterfly” and “Miss Saigon;” “Little Women” and “Little Women;” “Cendrillon” and “Into the {…}