samples/poppopwin: upgrade to first-class sample

samples/pop_pop_win/web/_audio.dart

 part of game;
 
 class _Audio {
   static const List<String> _AUDIO_NAMES =
       const ['Pop0', 'Pop1', 'Pop2', 'Pop3', 'Pop4', 'Pop5', 'Pop6', 'Pop7', 'Pop8',
              'Bomb0', 'Bomb1', 'Bomb2', 'Bomb3', 'Bomb4',
              GameAudio.THROW_DART, GameAudio.FLAG, GameAudio.UNFLAG, GameAudio.CLICK, GameAudio.WIN];
 
   final AudioLoader _audioLoader;
   final Map<String, AudioBuffer> _buffers = new Map();
@@ skipping 102 matching lines @@
         }
       }
     } catch (e) {
       print("Error getting client info: ${e}");
     }
     return null;
   }
 
   static String _getAudioPath(String name) {
     assert(_audioFormat != null);
-    return 'audio/${_audioFormat}/$name.${_audioFormat}';
+    return '${_ASSET_DIR}audio/${_audioFormat}/$name.${_audioFormat}';
   }
 
   static Iterable<String> _getAudioPaths(Iterable<String> names) {
     return names.map(_getAudioPath);
   }
 }