code: private static Node normalizeAssignmentOp(Node n) {
      Node lhs = n.getFirstChild();
      Node rhs = n.getLastChild();
      Node newRhs = new Node(NodeUtil.getOpFromAssignmentOp(n),
          lhs.cloneTree(), rhs.cloneTree()).srcrefTree(n);
      return replace(n, IR.assign(lhs.cloneTree(), newRhs).srcrefTree(n));
    }
docs: Transforms `a += b` to `a = a + b`.