anchor.diff:    bool get movable => _flags & MOVABLE != 0;
   }
   
  -class TerrainDB {
  +/// 地形テーブル
  +class TerrainTable {
  +  static final Map<String, Terrain> _tabel = <String, Terrain> {
  +    '#': const Terrain.Block('#', 'Silver'),
  +    '.': const Terrain('.', 'Silver'),
  +  };
     static Terrain get(String symbol) {
  -    return const Terrain.Block('#', 'Silver');
  +    return _tabel[symbol];
     }
   }
   
  
positive.code:   Array2D<Actor> _actor;
  
    Stage(Size size) {
      _terrain = new Array2D<Terrain>(size, const Terrain('.', 'silver'));
      _actor = new Array2D<Actor>(size);
    }
  
  
negative.code:   }
  
    void setTerrain(String symbol, Coordinate coordinate) {
      _terrain[coordinate] = TerrainTable.get(symbol);
    }
  }
  
  