Put 40 boys alone on an island and what do you get? Harmony or chaos? British author William Golding predicted not good things in his harrowing 1954 classic novel “Lord of the Flies,” and a new powerful, kinetic TV adaptation makes an inspired case that he was probably right. The Netflix series premiering Monday follows more than two dozen British boys in the mid-1950s stranded on a tropical island after a plane crash as they descend into tyranny and violence, making an indictment about the fragility of democracy and the shallow veneer of civilization. “We’ve advanced socially or we’ve advanced technologically, but those issues are still there,” says David McKenna, who plays a sensible boy nicknamed Piggy.…