It’s one of the most extraordinary design challenges ever: The Kennedy Center in Washington, DC, will present this summer productions of six musicals by Stephen Sondheim. The plan is to run three shows in repertory at a time. In May and June, there will be Company, Sweeney Todd, and Sunday in the Park With George. Following in July and August will be A Little Night Music, Merrily We Roll Along, and Passion. Designing all six of these productions is Derek McLane.