Updated old button references in dev/integration_tests/flutter_gallery ......

Updated old button references in dev/integration_tests/flutter_gallery ... transformation_demo (#63432)

dev/integration_tests/flutter_gallery/lib/demo/transformations/transformations_demo.dart

@@ -104,7 +104,7 @@ class _TransformationsDemoState extends State<TransformationsDemo> {
         ],
       ),
       actions: <Widget>[
-        FlatButton(
+        TextButton(
           child: const Text('OK'),
           onPressed: () {
             Navigator.of(context).pop();