samples/poppopwin: upgrade to first-class sample

samples/pop-pop-win/lib/src/html/high_score_view.dart

 part of ppw_html;
 
 class HighScoreView {
   final DivElement _div;
   final GameManager _manager;
 
   HighScoreView(this._manager, this._div) {
     assert(_div != null);
     assert(_manager != null);
 
     _manager.bestTimeUpdated.listen((args) => _update());
 
     _update();
   }
 
   void _update() {
     _manager.bestTimeMilliseconds
       .then((int milliseconds) {
         if(milliseconds != null) {
           final duration = new Duration(seconds: milliseconds ~/ 1000);
           _div.innerHtml = duration.toString();
         } else {
           _div.innerHtml = '';
         }
       });
   }
 }