Migrating a block of dart 1 tests to dart 2

tests/language_2/closure_cycles_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Based on dartbug.com/7681
 // Verify that context chains do not lead to unintended memory being held.
 
 library closure_cycles_test;
 
 import "dart:async";
 
@@ skipping 23 matching lines @@
     y.onY = () {
       y; // Capturing y can lead to endless context chains!
       doIt();
     };
   };
 }
 
 void main() {
   doIt();
 }