Clean up ng-flutter a bit (#2236)

No functional change, just while I was verifying it still works with the
latest Angular releases, completed some cleanup as well.

web_embedding/ng-flutter/flutter/lib/pages/counter.dart

@@ -1,13 +1,13 @@
 import 'package:flutter/material.dart';
 
 class CounterDemo extends StatefulWidget {
-  final ValueNotifier<int> counter;
-
   const CounterDemo({
     super.key,
     required this.counter,
   });
 
+  final ValueNotifier<int> counter;
+
   @override
   State<CounterDemo> createState() => _CounterDemoState();
 }