samples/poppopwin: upgrade to first-class sample

samples/pop_pop_win/lib/src/html/game_storage.dart

 part of ppw_html;
 
 class GameStorage {
   static const _gameCountKey = 'gameCount';
   final EventHandle _bestTimeUpdated = new EventHandle();
   final Map<String, String> _cache = new Map<String, String>();
 
   Future<int> get gameCount => _getIntValue(_gameCountKey);
 
   Stream get bestTimeUpdated => _bestTimeUpdated.stream;
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   static int _parseValue(String value, int defaultValue) {
     if(value == null) {
       return defaultValue;
     } else {
       return int.parse(value);
     }
   }
 
 }