anchor.diff:      expect(find.text('Add'), findsNothing);
   
       // Test hover for tooltip.
  -    TestGesture? gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
  -    await gesture!.addPointer();
  -    addTearDown(() => gesture?.removePointer());
  +    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
  +    await gesture.addPointer();
  +    addTearDown(() => gesture.removePointer());
       await gesture.moveTo(tester.getCenter(find.byType(FloatingActionButton)));
       await tester.pumpAndSettle();
   
       expect(find.text('Add'), findsOneWidget);
   
       await gesture.moveTo(Offset.zero);
  -    await gesture.removePointer();
  -    gesture = null;
       await tester.pumpAndSettle();
   
       expect(find.text('Add'), findsNothing);
  
positive.code:     expect(find.text('Add'), findsOneWidget);
  
      await gesture.moveTo(Offset.zero);
      await gesture.removePointer();
      gesture = null;
      await tester.pumpAndSettle();
  
      expect(find.text('Add'), findsNothing);
  
negative.code:     expect(find.text('Add'), findsNothing);
  
      // Test hover for tooltip.
      final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
      await gesture.addPointer();
      addTearDown(() => gesture.removePointer());
      await tester.pumpAndSettle();
      await gesture.moveTo(tester.getCenter(find.byType(FloatingActionButton)));
      await tester.pumpAndSettle();
  