xxxxxxxxxx
var uri = Uri.https('example.com', 'create');
var request = http.MultipartRequest('POST', uri)
..fields['user'] = 'nweiz@google.com'
..files.add(await http.MultipartFile.fromPath(
'package', 'build/package.tar.gz',
contentType: MediaType('application', 'x-tar')));
var response = await request.send();
if (response.statusCode == 200) print('Uploaded!');