Re-land "Extend the mDNS package with a native extension used on Mac OS"

pkg/mdns/lib/src/native_extension_api.dart

Index: pkg/mdns/lib/src/native_extension_api.dart
diff --git a/samples/github/ios/github/AppDelegate.h b/pkg/mdns/lib/src/native_extension_api.dart
similarity index 56%
copy from samples/github/ios/github/AppDelegate.h
copy to pkg/mdns/lib/src/native_extension_api.dart
index ae36cece8c0eb86047f3d317b4f5aedd20805dc3..cc9de846348c9e64d51d5e613e227f986f1a2221 100644
--- a/samples/github/ios/github/AppDelegate.h
+++ b/pkg/mdns/lib/src/native_extension_api.dart
@@ -2,10 +2,10 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
-#import <UIKit/UIKit.h>
+library mdns.src.native_extension_api;
 
-@interface AppDelegate : UIResponder <UIApplicationDelegate>
+import 'dart:isolate';
 
-@property (strong, nonatomic) UIWindow* window;
+import "dart-ext:../native/mdns_extension_lib";
 
-@end
+SendPort servicePort() native 'MDnsExtension_ServicePort';