Makes the Flutter Gradle Plugin apply the `FlutterExtension` (the class that vends `flutter.minSdkVersion`, etc) to Flutter plugins.
This allows plugin authors to use `flutter.compileSdkVersion` and the like in plugin `build.gradle` files.
Doesn't use it in templates yet - holding off till we can see how this works in the plugins repo (which will sadly have to wait till this makes the next stable ð¢ )
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