Currently we don't have any of the APIs related to plugins being able to publish a value implemented for macOS:

https://github.com/flutter/engine/blob/2360950aead7b35767cc54e9682e58e7dcaab54d/shell/platform/darwin/ios/framework/Headers/FlutterPlugin.h#L328-L339

https://github.com/flutter/engine/blob/2360950aead7b35767cc54e9682e58e7dcaab54d/shell/platform/darwin/ios/framework/Headers/FlutterPlugin.h#L414-L422

These APIs are used in webview_flutter, so not having them will require otherwise-unnecessary ifdefing (and as we continue to consolidate iOS and macOS engine headers, extra ifdefs there as well). I believe they should be easy to wire up.