myip.dart

class MyIp {
  MyIp(this._client);
  final Client _client;

  Future<String> get ipAddress async {
    final apiResult = await _client.get('https://api.ipify.org/?format=json');
    return json.decode(apiResult.body)['ip'];
  }
}

// Testing code

test('it get my ip', () async {
  final fakeClient = MockClient((_) async =>
    Response(json.encode({"ip": "192.168.0.1"}), 200)
  );
  final myIp = MyIp(fakeClient);

  expect(await myIp.ipAddress, "192.168.0.1");
});