Multi-pre-select from template is either impossible or not documented about paper-listbox HOT 4 OPEN

    //...userList was loaded from server via REST

    //clear listbox items
    PolymerDom listboxDom = Polymer.dom(userMultiChoice);
    for (Node childNode in listboxDom.childNodes)
      listboxDom.removeChild(childNode);

    //recreate listbox items
    List<int> selectedItems = new List<int>();
    for (int i = 0; i < userList.length; i++) {
      User user = userList[i];

      for (String username in selectedUsernames) {//~~~~
        if (username == user.username) {
          selectedItems.add(i);
          break;
        }
      }

      PaperItem item = new PaperItem();
      item.text = user.username;
      listboxDom.append(item);
    }

    //apply selection
    for (int item in selectedItems) {
      userMultiChoice.select(item);
    }