notify(key, oldValue) {
    let eventType;
    eventType = `change:${key}`;
    if (this.hasListener(eventType)) {
      this.dispatchEvent(new ObjectEvent(eventType, key, oldValue));
    }
    eventType = ObjectEventType.PROPERTYCHANGE;
    if (this.hasListener(eventType)) {
      this.dispatchEvent(new ObjectEvent(eventType, key, oldValue));
    }
  }
@param {string} key Key name.
@param {*} oldValue Old value.