A key Walgreen Co. revenue gauge slid more than analysts expected in December. The Deerfield, Ill., company said revenue from stores open at least a year dropped 6.1 percent last month as a shift toward generic drugs and a calendar quirk hurt sales. Pharmacy revenue tumbled 8.9 percent, while revenue from the rest of the stores fell 2.3 percent. Analysts had expected a slightly larger-than-reported drop of 9 percent drop in pharmacy revenue.