fix aria role for expanded (do not use "undefined")

src/vs/base/parts/tree/browser/treeView.ts

@@ -215,7 +215,7 @@ export class ViewItem implements IViewItem {
 			this.element.removeAttribute('aria-labelledby');
 		}
 		if (this.model.hasChildren()) {
-			this.element.setAttribute('aria-expanded', String(this.model.isExpanded()));
+			this.element.setAttribute('aria-expanded', String(!!this.model.isExpanded()));
 		} else {
 			this.element.removeAttribute('aria-expanded');
 		}

src/vs/editor/contrib/find/browser/findWidget.ts

@@ -893,7 +893,7 @@ class SimpleButton extends Widget {
 	}
 
 	public setExpanded(expanded: boolean): void {
-		this._domNode.setAttribute('aria-expanded', String(expanded));
+		this._domNode.setAttribute('aria-expanded', String(!!expanded));
 	}
 
 	public toggleClass(className: string, shouldHaveIt: boolean): void {