You have to assign an onLoad callback before you call send. I'm not sure what you mean with only inside an onReadyStateChange. Maybe you want to assign the responseText to a variable outside the the callback.
Create a method:
Future<String> send(String method, String url, String infojson) {
var completer = new Completer<String>();
// var result;
req=new HttpRequest()
..open(method,url)
..onLoad.listen((event) {
//print('Request complete ${event.target.reponseText}'))
// result = event.target.responseText;
completer.complete(event.target.responseText);
})
..send(infojson);
return completer.future;
}
and call this method like
var result;
send(method, url).then(
(e) {
// result = e;
print('Request complete ${e}'));
});