The heart of retail in New England is about to shift westward. General Growth Properties, Inc. (NYSE: GGP) today confirmed details concerning the Sept. 7 opening of its highly anticipated addition to Natick Collection. When doors to the center's 550,000-square-foot expansion open, Natick Collection will offer six department stores including Massachusetts' first Nordstrom and first suburban Neiman Marcus and 270 luxury retailers and restaurants. In all, the complex will span 1.7 million square