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

test/performance/perf.dart

 import 'dart:async';
 import 'dart:html';
 import 'package:widget/effects.dart';
 
 void main() {
 
   _initialize();
   query('#do-swap').onClick.listen(_doSwap);
 
 }
@@ skipping 49 matching lines @@
 
   final newActive = _contentElementField.children[index];
   return showItem(newActive, effect: effect, duration: duration, effectTiming: effectTiming, hideEffect: hideEffect);
 }
 
 Future<bool> showItem(Element item, {ShowHideEffect effect, int duration, EffectTiming effectTiming, ShowHideEffect hideEffect}) {
   assert(_contentElementField.children.contains(item));
 
   final oldActiveChild = _activeItem;
   if(oldActiveChild == item) {
-    return new Future<bool>.immediate(true);
+    return new Future<bool>.value(true);
   }
 
   [oldActiveChild, item].forEach((e) => e.classes.remove(_dirClassPrev));
 
   oldActiveChild.classes.remove(_activeClass);
   oldActiveChild.classes.add(_dirClassPrev);
 
   item.classes.add(_activeClass);
 
   return Swapper.swap(_contentElementField, item, effect: effect, duration: duration, effectTiming: effectTiming, hideEffect: hideEffect)
@@ skipping 26 matching lines @@
 
     // A bit of a hack. Because we call Swap w/ two displayed items:
     // one marked 'prev' and one marked 'next', Swap tries to hide one of them
     // this only causes a problem when clicking right the first time, since all
     // times after, the cached ShowHideState of the item is set
     // So...we're going to walk the showHide states of all children now
     // ...and ignore the result...but just to populate the values
     theItems.forEach((f) => ShowHide.getState(f));
   }
 }