“Your new neighbors”: Few phrases evoke such a mix of hope and dread. They’ll provide barbecues and babysitters, or else they’ll be the occasion for 911 calls and “Clean It or Lien It” signs. Karen Zacarias’s play Native Gardens, which recently ran at Washington DC’s Arena Stage, uses the new neighbors as a microcosm of social and demographic change. Zacarias wants to offer hope of reconciliation in an increasingly divided country. But reconciliation turns out to be much harder to make plausible than division.