On a mild spring night in Chicago, a woman told her 18-year-old boyfriend she wanted money for a barbecue. He rounded up three teenage friends, each with a long criminal record, and, according to prosecutors , they donned masks, carried guns, and robbed four people, tossing two to the ground. They went searching for more victims in a stolen Kia; shortly after 1:30 a.m. they crossed paths with Aréanah Preston. Preston, a police officer, had finished her shift and, still in uniform, parked across