code: function(x, y, range, visibility){
              this.fovMap.set(x, y, visibility);
              if(visibility){
                  var tile = this.game.map.get(x, y);
                  if(tile){
                      var key = x + ',' + y;
                      // check for duplicates
                      if(this.visibleTileKeys.indexOf(key) === -1){
                          this.visibleTiles.push({
                              x: x,
                              y: y,
                              value: tile,
                              range: range
                          });
                          this.visibleTileKeys.push(key);
                      }
                  }
              }
          }
docs: Sets the visibility of a checked map tile
  @method setMapTileVisible
  @param {Number} x - The map coord position to set.
  @param {Number} y - The map coord position to set.
  @param {Number} range - The distance from this fov origin.
  @param {Number} visibility - The visibility of this tile coord.