The css in `assets/treeview.css` doesn't work. It refers to a file `treeview-sprite.gif` which doesn't exist (at least not anymore). I have a copy of yui 2.5.1 and the file has different contents (it refers to the file `sprite-orig.gif` which does exist). Replacing the version in 2.6.0 with the version in 2.5.1 solves the problem.
