test_async.dart

import 'dart:async';
void main() {
  Tester a = Tester();
  a.test();

}

class Tester {
  Object _currentRouterDelegateTransaction;
  void test() {
    print('begin transaction $_currentRouterDelegateTransaction');
    _createFuture()
      .then<void>(_verifyRouteNameParserStillCurrent)
      .then<void>(_verifyRouterDelegateStillCurrent(_currentRouterDelegateTransaction))
      .then<void>((void data) {
      print('should not reach here');
    });
  }


  Future<void> _createFuture() {
    return Future<void>.value(null);
  }

  Future<void> _verifyRouteNameParserStillCurrent(void data) {
    _currentRouterDelegateTransaction = Object();
    return Future<void>.value();
  }

  Function _verifyRouterDelegateStillCurrent(Object transaction) {
    return (void data) {
      // this should fail.
      assert (transaction == _currentRouterDelegateTransaction);
      return Future<void>.value();
    };
  }
}

typedef _AsyncPassthrough<Q> = Future<Q> Function(Q);