Update `platform_channels` for 3.10 (#1902)

Replaces deprecated APIs in unit tests.

Fixes https://github.com/flutter/samples/issues/1765

## 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 `///`).
- [x] All existing and new tests are passing.

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

platform_channels/test/src/event_channel_demo_test.dart

@@ -20,7 +20,8 @@ void main() {
       // and add the incoming message to the StreamController used by the EventChannel
       // after decoding the message with codec used by the EventChannel.
       void emitValues(ByteData? event) {
-        ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+        TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+            .handlePlatformMessage(
           'eventChannelDemo',
           event,
           (reply) {},
@@ -29,7 +30,8 @@ void main() {
 
       // Register a mock for EventChannel. EventChannel under the hood uses
       // MethodChannel to listen and cancel the created stream.
-      ServicesBinding.instance.defaultBinaryMessenger
+
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
           .setMockMessageHandler('eventChannelDemo', (message) async {
         // Decode the message into MethodCallHandler.
         final methodCall = standardMethod.decodeMethodCall(message);

platform_channels/test/src/pet_list_screen_test.dart

@@ -26,15 +26,18 @@ void main() {
 
     setUpAll(() {
       // Mock for the pet list received from the platform.
-      basicMessageChannel.setMockMessageHandler((message) async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockDecodedMessageHandler(basicMessageChannel, (message) async {
         petListModel = PetListModel.fromJson(message!);
         return null;
       });
 
       // Mock for the index received from the Dart to delete the pet details,
       // and send the updated pet list back to Dart.
-      const BasicMessageChannel<ByteData?>('binaryCodecDemo', BinaryCodec())
-          .setMockMessageHandler((message) async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockDecodedMessageHandler(
+              const BasicMessageChannel<ByteData?>(
+                  'binaryCodecDemo', BinaryCodec()), (message) async {
         // Convert the ByteData to String.
         final index = utf8.decoder.convert(message!.buffer
             .asUint8List(message.offsetInBytes, message.lengthInBytes));