If you think you can’t dance to Mozart’s music with 21st century abandon the same way you can dance at your favorite D.C. club, you simply must see the Washington National Opera’s current “Cosi fan tutti,” a superb contemporary production set “yesterday afternoon in Washington, D.C.” In the very first scene, its two major female characters, the sisters Fiordiligi (Elizabeth Futral) and Dorabella (Renata Pokupi), express their delight at being in love by swinging and swaying to Mozart’s music, cell phones in hand. The libretto (by Lorenzo da Ponte) and music are still the original; the modern steps fit right in.