constructor() {
        super();
        this._updateState = 0;
        this._instanceProperties = undefined;
        this._updatePromise = microtaskPromise;
        this._hasConnectedResolver = undefined;
        /**
         * Map with keys for any properties that have changed since the last
         * update cycle with previous values.
         */
        this._changedProperties = new Map();
        /**
         * Map with keys of properties that should be reflected when updated.
         */
        this._reflectingProperties = undefined;
        this.initialize();
    }
Base element class which manages element properties and attributes. When
properties change, the `update` method is asynchronously called. This method
should be supplied by subclassers to render updates as desired.