When Boston’s Paul Pierce got free for a breakaway dunk with about four minutes left Tuesday night, it was clear the Warriors’ home dominance over the Celtics was over. The dunk was the backbreaking blow in a 115-93 loss, the Warriors’ first at home against the Celtics in a surprising six-game streak that began before the formation of Boston’s Big Three — Ray Allen, Paul Pierce and Kevin Garnett. The loss also halted a three-game overall winning streak for the Warriors (26-30).