update(_changedProperties) {
        if (this._reflectingProperties !== undefined &&
            this._reflectingProperties.size > 0) {
            // Use forEach so this works even if for/of loops are compiled to for
            // loops expecting arrays
            this._reflectingProperties.forEach((v, k) => this._propertyToAttribute(k, this[k], v));
            this._reflectingProperties = undefined;
        }
    }
Updates the element. This method reflects property values to attributes.
It can be overridden to render and keep updated element DOM.
Setting properties inside this method will *not* trigger
another update.

* @param _changedProperties Map of changed properties with old values