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

lib/src/effects/css3_transition_effect.dart

 part of effects;
 
 class Css3TransitionEffect extends ShowHideEffect {
   static const List<String> _reservedProperties = const ['transitionProperty', 'transitionDuration'];
   final String _property;
   final String _hideValue;
   final String _showValue;
   final Map<String, String> _animatingOverrides;
 
   Css3TransitionEffect(this._property, this._hideValue, this._showValue, [Map<String, String> animatingOverrides]) : _animatingOverrides = animatingOverrides == null ? new Map<String, String>() : new Map<String, String>.from(animatingOverrides) {
@@ skipping 60 matching lines @@
 
   void _setShowValue(Element element, String value, int desiredDuration, EffectTiming timing) {
     final cssTimingValue = CssEffectTiming._getCssValue(timing);
 
     element.style.transitionTimingFunction = cssTimingValue;
     element.style.transitionProperty = _property;
     element.style.transitionDuration = '${desiredDuration}ms';
     element.style.setProperty(_property, value);
   }
 
-  static Map<String, String> _recordProperties(Element element, Collection<String> properties) {
+  static Map<String, String> _recordProperties(Element element, Iterable<String> properties) {
     final map = new Map<String, String>();
 
     for(final p in properties) {
       assert(!map.containsKey(p));
       map[p] = element.style.getPropertyValue(p);
     }
 
     return map;
   }
 }
@@ skipping 32 matching lines @@
 
   static Map<String, String> cleanup(Element element) {
     final value = _values[element];
     assert(value != null);
     _values[element] = null;
     return value._cleanup();
   }
 }