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

lib/components/accordion.dart

Index: lib/components/accordion.dart
diff --git a/lib/components/accordion.dart b/lib/components/accordion.dart
index d82192aa41f4e2439635e06e7f200c59db9d89fc..5a68b8202af32d8459e3e032ae2f4c4c0985441a 100644
--- a/lib/components/accordion.dart
+++ b/lib/components/accordion.dart
@@ -26,15 +26,12 @@ class Accordion extends WebComponent {
       });
   }
 
-  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);
     }
   }