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

lib/components/tabs.dart

Index: lib/components/tabs.dart
diff --git a/lib/components/tabs.dart b/lib/components/tabs.dart
index 11b5ca11912ce7197f2fd57b279df6649e9b53b2..f96c3bfa5372833529c42691c775d4a320cedc61 100644
--- a/lib/components/tabs.dart
+++ b/lib/components/tabs.dart
@@ -92,7 +92,7 @@ class Tabs extends WebComponent {
     return target;
   }
 
-  List<Element> _getAllTabs() => this.queryAll('x-tabs > .nav-tabs > li');
+  List<Element> _getAllTabs() => this.queryAll('[is=x-tabs] > .nav-tabs > li');
 
   void _ensureAtMostOneTabActive() {
     final tabs = _getAllTabs();
@@ -114,7 +114,7 @@ class Tabs extends WebComponent {
   }
 
   SwapComponent _getSwap() {
-    final Element element = this.query('x-tabs > x-swap');
+    final Element element = this.query('[is=x-tabs] > [is=x-swap]');
     if(element != null) {
       if(element is SwapComponent) {
         return element;