loadTile(tile, opt_priority) {
    tile.sourceUrl =
        this.tilesKey_.formatTileUrl(tile.coord.x, tile.coord.y, tile.zoom);
    tile.startLoad();
  }
Asynchronously loads the tile's source data.
@param {!AbstractTile} tile The tile to load.
@param {number=} opt_priority The priority of the tile.
@override