Update dartpad example project to use Notifier instead of StateNotifier

[Also filed dart-lang/dart-pad/issues/2762 in case they're the ones who should update this.]

Riverpod sample project in dartpad.dev uses StateNotifier and ref.read(counterProvider.notifier).state++. It should be updated to use Notifier since that's the current state-of-the-art (according to updated docs):

with Riverpod 2.0, new classes were introduced: Notifier / AsyncNotifer. StateNotifier is now discouraged

Notifier's getter and setter for state are marked as @protected and @visibleForTesting, so the current DartPad sample is promoting bad practice.

I believe this should be the new sample code (I just copied the Notifier from example code in doc comment and changed .state++ to .increment()):

// Copyright (c) 2018, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';

// This is a reimplementation of the default Flutter application
// using riverpod.
void main() {
  runApp(
    // Adding ProviderScope enables Riverpod for the entire project
    const ProviderScope(child: MyApp()),
  );
}

/// Providers are declared globally and specify how to create a state
final counterProvider = NotifierProvider<Counter, int>(Counter.new);

class Counter extends Notifier<int> {
  @override
  int build() {
    // Inside "build", we return the initial state of the counter.
    return 0;
  }

  void increment() {
    state++;
  }
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        colorSchemeSeed: Colors.blue,
        useMaterial3: true,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends ConsumerWidget {
  const MyHomePage({super.key});

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Riverpod example'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('You have pushed the button this many times:'),
            Consumer(
              builder: (context, ref, _) {
                final count = ref.watch(counterProvider);
                return Text(
                  '$count',
                  key: const Key('counterState'),
                  style: Theme.of(context).textTheme.headlineMedium,
                );
              },
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        key: const Key('increment_floatingActionButton'),
        // The read method is a utility to read a provider without listening to it
        onPressed: () => ref.read(counterProvider.notifier).increment(),
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

Additional context

rrousselGit / riverpod

Update dartpad example project to use Notifier instead of StateNotifier #3226