Newark Avenue in Jersey City, New Jersey,has the look of an old AmericanMain Street. Tightly packed buildings are in good repair. Storefront windows facing busy sidewalks are stocked with clothing and goods. Cars pull in and out of curbside parking spaces all day long; passersby recognize one another, stop, and chat on the street. It’s Rockwell-esque. Like most of America’s vibrant downtown retail districts, this one is run and sustained almost entirely by immigrants.