“American Dream,” the latest play by Japanese playwright-director Koki Mitani, takes its cue from a speech Dalton Trumbo, the screenwriter blacklisted during Hollywood’s Red Scare, gave in 1970: “It will do no good to search for villains or heroes or saints or devils because there were none; there were only victims.” The line sets the terms for the production, staged at Coex Artium from Friday to Sunday, as the final stop of its tour. It was the first of Mitani’s works to be performed in Korea i