function _getChildDirectories(treeData, projectRootPath) {
        var childDirectories = [];

        function addNodesAtDepth(treeData, parent, depth) {
            if (!treeData) {
                return;
            }

            treeData.forEach(function (value, key) {
                if (!isFile(value)) {
                    var directoryPath = key + "/";

                    childDirectories.push(directoryPath);
                }
            });
        }

        // start at the top of the tree and the first array element
        addNodesAtDepth(treeData, projectRootPath, 0);
        return childDirectories;
    }
@private
TODO: merge with _getOpenNodes?!
See `FileTreeViewModel.getChildNodes`.