Sandrine’s Bistro will close its doors on March 30, according to co-managing partner Gwen B. Trost. The French bistro, which was headed by award-winning chef Raymond E. Ost until August, has been a part of Harvard Square since 1996. Trost said that the restaurant had experienced a tough economic winter due to the harsh weather but stated the decision to close had been made before the snow storms had arrived.