// Download a file from Google Drive.
    Future downloadFile(drive.DriveApi api,
                        Client client,
                        String objectId,
                        String filename) {
      return api.files.get(objectId).then((drive.File file) {
        // The Drive API allows one to download files via `File.downloadUrl`.
        return client.readBytes(file.downloadUrl).then((bytes) {
          var stream = new File(filename).openWrite()..add(bytes);
          return stream.close();
        });
      });