Anna Pollak, 84, a mezzo-soprano associated for many years with London’s Sadler’s Wells opera company. The daughter of Austrian emigres, Pollak made her debut at Sadler’s Wells in 1945 as Dorabella in Mozart’s “Cosi fan Tutte.” She remained a member of the company until 1961, and returned frequently to sing specialist roles. She made her Royal Opera debut as Cherubino in Mozart’s “Le Nozze di Figaro” and at Glyndebourne as Dorabella in “Cosi,” both in 1952. She was awarded the Order of the British Empire in 1962. On Thursday in London.