Fix widget.dart to work with the latest web_ui library. Specifically, query selectors need to use "…

lib/components/accordion.dart

 import 'dart:html';
 import 'package:web_ui/web_ui.dart';
 import 'package:widget/effects.dart';
 import 'package:widget/widget.dart';
 import 'package:bot/bot.dart';
 
 /**
  * [Accordion] wraps a set of [Collapse] elements and ensures only one is visible
  * at a time.
  *
  * See [Collapse] for details on how content is interpreted.
  */
 class Accordion extends WebComponent {
   @protected
   void created() {
     ShowHideComponent.toggleEvent.forTarget(this).listen(_onOpen);
   }
 
   @protected
   void inserted() {
     // collapse all 'collapse' children
     _getAllCollapseElements()
       .map((Element e) => e.xtag)
       .forEach((ShowHideComponent e) {
         e.hide();
       });
   }
 
-  List<Element> _getAllCollapseElements() => this.queryAll('x-accordion > x-collapse');
+  List<Element> _getAllCollapseElements() =>
+      this.queryAll('[is=x-accordion] > [is=x-collapse]');
 
   void _onOpen(Event openEvent) {
-    if(openEvent.target is UnknownElement) {
-      final UnknownElement target = openEvent.target;
-      final ShowHideComponent shc = target.xtag as ShowHideComponent;
-      if(shc != null) {
-        _onShowHideToggle(shc);
-      }
+    Element target = openEvent.target;
+    if (target.xtag is ShowHideComponent) {
+      _onShowHideToggle(target.xtag);
     }
   }
 
   void _onShowHideToggle(ShowHideComponent shc) {
-    if(shc.isShown) {
+    if (shc.isShown) {
       _getAllCollapseElements()
         .map((Element e) => e.xtag)
         .where((e) => e != shc)
         .forEach((ShowHideComponent e) {
           e.hide();
         });
     }
   }
 }