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

lib/components/modal.dart

 import 'dart:async';
 import 'dart:html';
 import 'package:web_ui/web_ui.dart';
 import 'package:bot/bot.dart';
 import 'package:widget/effects.dart';
 import 'package:widget/widget.dart';
 
 // TODO: ESC to close: https://github.com/kevmoo/widget.dart/issues/17
 
 /**
@@ skipping 50 matching lines @@
   }
 
   @protected
   void inserted() {
     final modal = _getModalElement();
     if(modal != null && !isShown) {
       ModalManager.hide(modal);
     }
   }
 
-  Element _getModalElement() => this.query('x-modal > .modal');
+  Element _getModalElement() => this.query('[is=x-modal] > .modal');
 
   void _onClick(MouseEvent event) {
     if(!event.defaultPrevented) {
       final Element target = event.target as Element;
       if(target != null && target.dataset['dismiss'] == 'modal') {
         hide();
       }
     }
   }
 
   void _onBackdropClicked() {
     // TODO: ignoring some edge cases here
     // like what if this element has been removed from the tree before the backdrop is clicked
     // ...etc
     hide();
   }
 }