anchor.diff:  }
   
   double appBarHeight(WidgetTester tester) => tester.getSize(find.byType(AppBar)).height;
  -double appBarTop(WidgetTester tester) => tester.getTopLeft(find.byType(AppBar)).y;
  -double appBarBottom(WidgetTester tester) => tester.getBottomLeft(find.byType(AppBar)).y;
  +double appBarTop(WidgetTester tester) => tester.getTopLeft(find.byType(AppBar)).dy;
  +double appBarBottom(WidgetTester tester) => tester.getBottomLeft(find.byType(AppBar)).dy;
   
   double tabBarHeight(WidgetTester tester) => tester.getSize(find.byType(TabBar)).height;
   
  
positive.code: 
      // Drag the scrollable up and down. The app bar should not snap open, its
      // height should just track the the drag offset.
      TestGesture gesture = await tester.startGesture(const Point(50.0, 256.0));
      await gesture.moveBy(const Offset(0.0, 128.0)); // drag the appbar all the way open
      await tester.pump();
      expect(appBarTop(tester), 0.0);
  
negative.code:     // Now that the appbar is fully expanded, Perform the same drag
      // scenario in reverse: drag the appbar up and down and then trigger
      // the snap closed animation.
      gesture = await tester.startGesture(const Offset(50.0, 256.0));
      await gesture.moveBy(const Offset(0.0, -128.0));
      await tester.pump();
      expect(appBarBottom(tester), kTextTabBarHeight);
  