ANCHORAGE, Alaska — For the past five years, Alaska’s annual Iditarod sled dog race has gone off mostly free of controversy, as teams of dogs and their mushers braved the elements in the 1,000-mile (1,609-kilometer) test of endurance across the frozen wilderness.This year the deaths of three dogs during the race — and five more during training — have refocused attention on the darker side of Alaska’s state sport and raised questions about the ethics of asking animals to pull a heavy sled for hundreds of miles in subzero temperatures.Dog mushing has a long and storied tradition in…