_addOptionToValueDict(el, optionEl) {
      let index = Object.keys(this._valueDict).length;
      let key = this.dropdownOptions.id + index;
      let obj = {};
      optionEl.id = key;

      obj.el = el;
      obj.optionEl = optionEl;
      this._valueDict[key] = obj;
    }
Add option to value dict
@param {Element} el  original option element
@param {Element} optionEl  generated option element