iOS added the ability to more easily communicate between plugins in this PR, but neglected to add the same functionality for macOS.

For plugins that target both macOS and iOS through the shared darwin folder (including the webview_flutter plugin that this feature was meant to be used by) this can cause compatibility issues. (See this comment)

To correct this, we would like to add the FlutterPluginRegistrar.valuePublished(byPlugin:) to macOS.