Several of our 1P plugins present view controllers over the Flutter view; GIDSignIn needs this information, for example. Currently, we do this using UIApplication.sharedApplication.keyWindow, but in iOS 13 keyWindow is deprecated because of the introduction of scenes on iPads.

To make Flutter scene-friendly, we should provide an alternate way of getting that information. On the macOS embedding, for instance, I added a way to get the NSView of the Flutter content via the plugin registrar, which allows plugins to get the window they are part of for things like showing sheets. (I keep finding 3P plugins that ignore that and use the key window instead, but 🤷🏻 ). We should consider doing the same for iOS, or providing some alternate solution.

/cc @jmagman