login method
Implementation
Future<GoogleplusPayload> login() {
Completer<GoogleplusPayload> completer = Completer();
GooglePlusApi.login(
_options,
allowInterop((dynamic result) => completer.complete(_parseJsPayload(result))),
allowInterop((int codeError) => completer.completeError(codeError)));
return completer.future;
}