code: function(layer, position, bounds, url, size, options) {
          this.layer = layer;
          this.position = position.clone();
          this.setBounds(bounds);
          this.url = url;
          if (size) {
              this.size = size.clone();
          }
  
          //give the tile a unique id based on its BBOX.
          this.id = OpenLayers.Util.createUniqueID("Tile_");
  
          OpenLayers.Util.extend(this, options);
  
          this.events = new OpenLayers.Events(this);
          if (this.eventListeners instanceof Object) {
              this.events.on(this.eventListeners);
          }
      }
docs: TBD 3.0 -- remove 'url' from the list of parameters to the constructor.
  there is no need for the base tile class to have a url.
   
  Constructor: OpenLayers.Tile
  Constructor for a new <OpenLayers.Tile> instance.
  
  Parameters:
  layer - {<OpenLayers.Layer>} layer that the tile will go in.
  position - {<OpenLayers.Pixel>}
  bounds - {<OpenLayers.Bounds>}
  url - {<String>}
  size - {<OpenLayers.Size>}
  options - {Object}