code: function (file_system) {
                  if (file_system) {
                      if (successCallback) {
                          fileSystems.getFs(file_system.name, function (fs) {
                              // This should happen only on platforms that haven't implemented requestAllFileSystems (windows)
                              if (!fs) {
                                  fs = new FileSystem(file_system.name, file_system.root);
                              }
                              successCallback(fs);
                          });
                      }
                  } else {
                      // no FileSystem object returned
                      fail(FileError.NOT_FOUND_ERR);
                  }
              }
docs: if successful, return a FileSystem object