int contentLength

The total length of the request body, in bytes. This is calculated from fields and files and cannot be set manually.

Source 

@override
int get contentLength {
  int length = 0;

  fields.forEach((String name, String value) {
    length += "--".length + _BOUNDARY_LENGTH + "\r\n".length +
        UTF8.encode(_headerForField(name, value)).length +
        UTF8.encode(value).length + "\r\n".length;
  });

  for (MultipartFile file in _files) {
    length += "--".length + _BOUNDARY_LENGTH + "\r\n".length +
        UTF8.encode(_headerForFile(file)).length +
        file.length + "\r\n".length;
  }

  return length + "--".length + _BOUNDARY_LENGTH + "--\r\n".length;
}
void contentLength=(int value)

Source 

@override
set contentLength(int value) {
  throw new UnsupportedError("Cannot set the contentLength property of "
      "multipart requests.");
}