At virtually no cost, debt-ridden Berlin got the chance last month to “show one of the most distinguished private collections of its kind in the world — one that, moreover, has never been shown in its entirety to the public. Yet the collection has been unwelcome elsewhere because it belongs to Friedrich Christian Flick, the multimillionaire heir of a leading Nazi arms manufacturer. And for Berlin, a city that has served variously as the epicenter of the avant-garde and Nazi despotism, that fact presents a troubling dilemma.”