recapthca3 and flutter web

recaptcha.dart

@JS()
library grecaptcha;

import 'package:js/js.dart';
import 'package:js/js_util.dart';

@JS()
@anonymous
class Options {
  external String get action;

  // Must have an unnamed factory constructor with named arguments.
  external factory Options({String action});
}


@JS('grecaptcha.ready')
external captchaReady(Function callback);

@JS("grecaptcha.execute")
external captchaExecute(String key, Options opts);

dynamic doValidation(String key) async {
  var promise = captchaExecute(key, Options(action: 'submit'));
  var qs = await promiseToFuture(promise);
  return qs;
}

screen.dart

 RaisedButton(
    child: Text('Submit'),
     onPressed: () {
          doValidation(yourCaptchaKey).then((data) {
            var token = data.toString();
            if (validateTokenOnServer(token)) {
              
            }
          });
     })

servervalidation.cs

 private bool VerifyCaptchaResponse(string response)
        {
            var json = "https://www.google.com/recaptcha/api/siteverify"
                .AddQueryParam("secret",yoursecret)
                .AddQueryParam("response", response).GetJsonFromUrl();

            var data =JSON.parse(json).ToObjectDictionary();
            return data["success"].ConvertTo<bool>() && data["score"].ConvertTo<float>() > .3;

        }