Update for Flutter 3.10 beta (#1746)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

---------

Co-authored-by: David Iglesias <ditman@gmail.com>
Co-authored-by: Mark Thompson <2554588+MarkTechson@users.noreply.github.com>
Co-authored-by: John Ryan <ryjohn@google.com>

platform_channels/test/src/method_channel_demo_test.dart

@@ -9,23 +9,20 @@ import 'package:platform_channels/src/method_channel_demo.dart';
 
 void main() {
   group('MethodChannelDemo tests', () {
-    setUpAll(() {
-      const methodChannel = MethodChannel('methodChannelDemo');
-
-      // Register a mock MethodCallHandler.
-      methodChannel.setMockMethodCallHandler((call) async {
-        var count = call.arguments['count'] as int;
-        if (call.method == 'increment') {
-          return ++count;
-        } else if (call.method == 'decrement') {
-          return --count;
-        }
-
-        return MissingPluginException();
-      });
-    });
-
     testWidgets('MethodChannelDemo count test', (tester) async {
+      tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(
+        const MethodChannel('methodChannelDemo'),
+        (call) async {
+          var count = call.arguments['count'] as int;
+          if (call.method == 'increment') {
+            return ++count;
+          } else if (call.method == 'decrement') {
+            return --count;
+          }
+
+          return MissingPluginException();
+        },
+      );
       await tester.pumpWidget(const MaterialApp(
         home: MethodChannelDemo(),
       ));