anchor.diff:          return a + translation.y;
         });
       case SharedAxisTransitionType.scaled:
  -      // SharedAxisTransitionType.scaled should not return a translation
  -      // offset.
  -      return null;
  +      assert(
  +        false,
  +        'SharedAxisTransitionType.scaled does not have a translation offset',
  +      );
  +      return 0.0;
     }
   }
   
  
positive.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,
  
negative.code: 
          // Top route is fading/coming in.
          expect(find.text(topRoute), findsOneWidget);
          final double halfwayTopOffset = _getTranslationOffset(
            topRoute,
            tester,
            SharedAxisTransitionType.vertical,
  