anchor.diff:  
           // Top route is fading/coming in.
           expect(find.text(topRoute), findsOneWidget);
  -        final double? halfwayTopOffset = _getTranslationOffset(
  +        final double halfwayTopOffset = _getTranslationOffset(
             topRoute,
             tester,
             SharedAxisTransitionType.horizontal,
  
positive.code:   });
  }
  
  double? _getTranslationOffset(
    String key,
    WidgetTester tester,
    SharedAxisTransitionType transitionType,
  
negative.code:         // Bottom route is fully faded out.
          expect(find.text(bottomRoute), findsOneWidget);
          expect(_getOpacity(bottomRoute, tester), 0.0);
          final double halfwayBottomOffset = _getTranslationOffset(
            bottomRoute,
            tester,
            SharedAxisTransitionType.horizontal,
  