code: function _mapError(err) {
          if (!err) {
              return null;
          }
  
          switch (err) {
          case appshell.fs.ERR_INVALID_PARAMS:
              return FileSystemError.INVALID_PARAMS;
          case appshell.fs.ERR_NOT_FOUND:
              return FileSystemError.NOT_FOUND;
          case appshell.fs.ERR_CANT_READ:
              return FileSystemError.NOT_READABLE;
          case appshell.fs.ERR_CANT_WRITE:
              return FileSystemError.NOT_WRITABLE;
          case appshell.fs.ERR_UNSUPPORTED_ENCODING:
              return FileSystemError.UNSUPPORTED_ENCODING;
          case appshell.fs.ERR_OUT_OF_SPACE:
              return FileSystemError.OUT_OF_SPACE;
          case appshell.fs.ERR_FILE_EXISTS:
              return FileSystemError.ALREADY_EXISTS;
          case appshell.fs.ERR_ENCODE_FILE_FAILED:
              return FileSystemError.ENCODE_FILE_FAILED;
          case appshell.fs.ERR_DECODE_FILE_FAILED:
              return FileSystemError.DECODE_FILE_FAILED;
          case appshell.fs.ERR_UNSUPPORTED_UTF16_ENCODING:
              return FileSystemError.UNSUPPORTED_UTF16_ENCODING;
          }
          return FileSystemError.UNKNOWN;
      }
docs: Convert appshell error codes to FileSystemError values.
  
  @param {?number} err An appshell error code
  @return {?string} A FileSystemError string, or null if there was no error code.
  @private