At its best, the annual Prototype Festival, coproduced by Beth Morrison Projects and HERE, whose 11th iteration runs through Sunday, revels in original forms and challenging subjects. “Angel Island,”Huang Ruo’s haunting oratorio, which had its New York premiere at BAM Harvey in Brooklyn last week, fit the bill, evoking the spirits of the hundreds of thousands of Asian people who arrived at San Francisco’s Angel Island during the first decades of the 20th century. Detained under harsh conditions for months or even years, a consequence of draconian 19th-century laws restricting Asian immigration to the U.S., they carved poems into the wooden walls of the barracks.