A sad au revoir in Cambridge: Sandrine’s Bistro (8 Holyoke St. at Massachusetts Avenue), a Harvard Square institution for 18 years — an eternity in the restaurant world — shutters on March 30. “It’s bittersweet,” says managing partner Gwen Trost, noting that bad winter weather affected business. An upscale restaurant from Classic Restaurant Concepts, which operates the Asgard pub in Cambridge and Boston’s Kinsale, will move in this spring. (Back Bay’s La Voile brasserie was slated to take the space, but the deal fell through.) “We’re looking at several interesting chefs and want to do something that complements the square, but not an Irish pub,” says chief operating officer Bill Goodwin, who aims to open for May graduations. Trost hopes to re-enter the world of hotel catering. “Those are my roots, and I’m looking for a job!” she laughs.