macos isolate platform channels (#110882)

Author: gaaclarke

dev/integration_tests/channels/lib/main.dart

@@ -173,7 +173,7 @@ class _TestAppState extends State<TestApp> {
     () => basicStringMessageToUnknownChannel(),
     () => basicJsonMessageToUnknownChannel(),
     () => basicStandardMessageToUnknownChannel(),
-    if (Platform.isIOS || Platform.isAndroid)
+    if (Platform.isIOS || Platform.isAndroid || Platform.isMacOS)
       () => basicBackgroundStandardEcho(123),
   ];
   Future<TestStepResult>? _result;

dev/integration_tests/channels/macos/Runner/MainFlutterWindow.swift

@@ -99,6 +99,13 @@ class MainFlutterWindow: NSWindow {
         binaryMessenger: registrar.messenger,
         codec: FlutterStandardMethodCodec(readerWriter: ExtendedReaderWriter())))
 
+    FlutterBasicMessageChannel(
+      name: "std-echo", binaryMessenger: registrar.messenger,
+      codec: FlutterStandardMessageCodec.sharedInstance()
+    ).setMessageHandler { message, reply in
+      reply(message)
+    }
+
     super.awakeFromNib()
   }