problem.dart

import 'dart:async';
import 'dart:collection';

void main(){
  final completer = new Completer<dynamic>();

  final future = completer.future;

  future.then((dynamic result){
    print('before error');
      LinkedHashSet<String> set = result as LinkedHashSet<String>;
    print('after error');
  });

  new Timer(const Duration(milliseconds: 400), (){
    completer.complete(new LinkedHashSet<dynamic>()..add('test'));
  });
}