Right or wrong, the code of the street in poor, largely black neighborhoods in the Bay Area is never, ever cooperate with police. To do so, the idea goes, means risking retaliation from criminals. The ethos cuts across generations, even as some who embrace it complain police do little about crime in their neighborhoods. The mind-set is moving from the streets to the mainstream, carried by rappers denouncing rats and T-shirts declaring, "Stop Snitchin." The T-shirts have proven popular in Oakland and elsewhere, prompting a debate within the communities where they're worn and frustrating police who say they're another...