function _markAsChanged(treeData, changed) {
        changed.forEach(function (filePath) {
            var objectPath = _filePathToObjectPath(treeData, filePath);
            if (objectPath) {
                treeData = treeData.updateIn(objectPath, _addTimestamp);
            }
        });
        return treeData;
    }
@private

Sets/updates the timestamp on the file paths listed in the `changed` array.

@param {Immutable.Map} treeData
@param {Array.<string>} changed list of changed project-relative file paths
@return {Immutable.Map} revised treeData