function addClassesProvider(callback) {
        return FileTreeView.addClassesProvider(callback);
    }
Adds a CSS class provider, invoked before each tree item is rendered.

@param {!function(!{name:string, fullPath:string, isFile:boolean}):?string} callback
* `name`: the file or directory name
* `fullPath`: full path to the file or directory
* `isFile`: true if it's a file, false if it's a directory
Return a string containing space-separated CSS class(es) to add, or undefined to leave CSS unchanged.