anchor.diff:  // and support multiple transition types
   const Duration _kTransitionDuration = const Duration(milliseconds: 150);
   const Point _kTransitionStartPoint = const Point(0.0, 75.0);
  -enum TransitionDirection { forward, reverse }
   class Transition extends AnimatedComponent {
     Transition({
       Key key,
  
positive.code: import 'package:sky/animation/animated_value.dart';
  import 'package:sky/animation/animation_performance.dart';
  import 'package:sky/animation/curves.dart';
  import 'package:sky/widgets/animated_component.dart';
  import 'package:sky/widgets/basic.dart';
  import 'package:sky/widgets/focus.dart';
  
negative.code:       Transition transition = new Transition(
          key: new Key.fromObjectIdentity(historyEntry),
          content: content,
          direction: (i <= state.historyIndex) ? Direction.forward : Direction.reverse,
          interactive: (i == state.historyIndex),
          onDismissed: () {
            setState(() {
  