code: function addFilePrefix( path, prefix ) {
    if ( typeof prefix === 'undefined' ) prefix = '@';
  
    var separatorPosition = path.lastIndexOf( '/' );
    if ( separatorPosition < 0 ) {
      // Let's try with Windows separators.
      separatorPosition = path.lastIndexOf( '\\' );
    }
    var filenameStart = separatorPosition > -1 ? separatorPosition + 1 : 0;
    var result = path.substr( 0, filenameStart ) + prefix + path.substr( filenameStart );
    return result.replace( /\\/g, '/' );
  }
docs: Add a prefix to  a filename. This is not as  simple as prepending the
  `prefix` to  the string `path`,  because `path` can  contain folders'
  separators. The  prefix must be prepended  to the real file  name and
  not to the whole path.
  Examples with `prefix` == "@":
  * `foobar.html`: `@foobar.html`
  * `myfolder/myfile.js`: `myfolder/@myfile.js`