constructor(type, key, oldValue) {
    super(type);

    /**
     * The name of the property whose value is changing.
     * @type {string}
     * @api
     */
    this.key = key;

    /**
     * The old value. To get the new value use `e.target.get(e.key)` where
     * `e` is the event object.
     * @type {*}
     * @api
     */
    this.oldValue = oldValue;
  }
@param {string} type The event type.
@param {string} key The property name.
@param {*} oldValue The old value for `key`.