code: function showSaveDialog(title, initialPath, proposedNewFilename, callback) {
          appshell.fs.showSaveDialog(title, initialPath, proposedNewFilename, _wrap(callback));
      }
docs: Display a save-file dialog and call back asynchronously with either a
  FileSystemError string or the path to which the user has chosen to save
  the file. If the dialog is cancelled, the path string will be empty.
  
  @param {string} title
  @param {string} initialPath
  @param {string} proposedNewFilename
  @param {function(?string, string=)} callback