xt;this.updateContents_();},set arrowVisible(val){if(this.arrowVisible_===val)return;this.arrowVisible_=!!val;this.updateContents_();},set tooltip(text){this.$.heading.title=text;},set selectionGenerator(generator){if(this.selectionGenerator_===generator)return;this.selectionGenerator_=generator;this.updateContents_();},get expanded(){return this.expanded_;},set expanded(expanded){if(this.expanded_===expanded)return;this.expanded_=!!expanded;this.updateContents_();},onHeadingDivClicked_(){this.dispatchEvent(new...