First the Shake Shack invasion, and now this: A rep tells us that the first Boston-area Smashburger opens at Natick's Sherwood Plaza on December 11. The Denver-based chain sells build-your-own burgers, Haagen Dazs shakes, salads, and local craft beer. There's also Boston-themed treats like a "New Englander" burger topped with cranberry sauce, grilled onions, and cranberry stilton. More Boston-area shops are planned. Now, if only In-n-Out would look eastward.