anchor.diff:    final double left;
   }
   
  -abstract class ModalRoute<T> extends LocalHistoryRoute<T> {
  +abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T> {
     ModalRoute({
       Completer<T> completer,
       this.settings: const NamedRouteSettings()
  -  }) : super(popCompleter: completer);
  +  }) : super.explicit(completer, null);
   
     // The API for general users of this class
   
  
positive.code:   }
  }
  
  abstract class LocalHistoryRoute<T> extends TransitionRoute<T> {
    LocalHistoryRoute({
      Completer<T> popCompleter,
      Completer<T> transitionCompleter
    }) : super(
      popCompleter: popCompleter,
      transitionCompleter: transitionCompleter
    );
  
    List<LocalHistoryEntry> _localHistory;
    void addLocalHistoryEntry(LocalHistoryEntry entry) {
      assert(entry._owner == null);
  
negative.code:     Completer<T> transitionCompleter
    }) : _popCompleter = popCompleter,
         _transitionCompleter = transitionCompleter;
  
    TransitionRoute.explicit(
      Completer<T> popCompleter,
      Completer<T> transitionCompleter
    ) : this(popCompleter: popCompleter, transitionCompleter: transitionCompleter);
  
    /// This future completes once the animation has been dismissed. For
    /// ModalRoutes, this will be after the completer that's passed in, since that
  