How do you use JS to open a Folder dialog

If you use this to open a dialog box to select a file:

     File.openDialog("Select your XML File", "*.xml")

The how do you open a dialog to select a folder.  I would think it would be something like

     Folder.openDialog

but alas, it is not, and I cannot find any reference on how to do it anywhere.  Does any one know?

Thanks