anchor.diff:   * Setup and teardown functions for a group and its parents, the latter
    * for chaining.
    */
  -class GroupContext {
  -  final GroupContext parent;
  +class _GroupContext {
  +  final _GroupContext parent;
   
     /** Description text of the current test group. */
     final String _name;
  
positive.code:  */
  void group(String description, void body()) {
    ensureInitialized();
    _currentContext = new GroupContext(_currentContext, description);
    try {
      body();
    } catch (e, trace) {
  
negative.code: // We use a 'dummy' context for the top level to eliminate null
  // checks when querying the context. This allows us to easily
  //  support top-level setUp/tearDown functions as well.
  final _rootContext = new _GroupContext();
  _GroupContext _currentContext = _rootContext;
  
  int _currentTestCaseIndex = 0;
  
  