addLabel_(label) {
    let oldLabels = this.labelSet_;

    let labelSet = new StringMap();
    this.labelSet_.forEach((k) => labelSet[k] = this.labelSet_[k]);
    labelSet.set(label.name, label);
    this.labelSet_ = labelSet;

    return oldLabels;
  }
Adds a label to the current label set. Returns the OLD label set.
@param {LabelState} label
@return {Object}