int contentLength

The size of the request body, in bytes. This is calculated from bodyBytes.

The content length cannot be set for Request, since it's automatically calculated from bodyBytes.

Source 

@override
int get contentLength => bodyBytes.length;
void contentLength=(int value)

Source 

@override
set contentLength(int value) {
  throw new UnsupportedError("Cannot set the contentLength property of "
      "non-streaming Request objects.");
}