anchor.diff:    asyncStart();
     asyncStart();
     StreamController c2 = new StreamController(onCancel: () {
  -    asyneEnd();
  +    asyncEnd();
       throw "ERROR2";
     });
     c2.add(1);
  
positive.code: void testCancelThrow() {
    asyncStart();
    asyncStart();
    StreamController c = new StreamController(onCancel: () {
      asyncEnd();
      throw "ERROR";
  
negative.code:   testCancelThrow();
    testCancelThrow2();
    testCancelThrow3();
    asyncEnd();
  }
  