getRequiredResources() {
    let requiredResources = RewardNode.prototype.getRequiredResources.call(this);
    requiredResources = requiredResources.concat(PKGS.getPkgForIdentifier('card_back_reward'));
    if (this._cardBackId != null) {
      const cardBackPkgId = PKGS.getCardBackPkgIdentifier(this._cardBackId);
      requiredResources = requiredResources.concat(PKGS.getPkgForIdentifier(cardBackPkgId));
    }
    return requiredResources;
  }
Returns a list of resource objects this node uses.
@returns {Array}