JERUSALEM -Masked Palestinians whirling slingshots clashed with Israeli riot police in two Arab neighborhoods of Jerusalem on Saturday after the shooting death of a teenage stone thrower. It was a sign of rising tensions on the eve of Palestinian commemorations of their uprooting during Israel's 1948 creation. The possibility of escalation comes at a critical time for U.S. Mideast policy. President Barack Obama's envoy to the region, George Mitchell, resigned Friday, and the U.S. president may now have to retool the administration's incremental approach to peacemaking. Obama is to deliver a Mideast policy speech in the coming week. Mitchell held...