unset(key, silent) {
    if (this.values_ && key in this.values_) {
      const oldValue = this.values_[key];
      delete this.values_[key];
      if (isEmpty(this.values_)) {
        this.values_ = null;
      }
      if (!silent) {
        this.notify(key, oldValue);
      }
    }
  }
Unsets a property.
@param {string} key Key name.
@param {boolean} [silent] Unset without triggering an event.
@api