[feature] SDK-specific dependencies

[dcharkes]

I'd like to be able to use packages in multiple SDKs, and like to be able to add support for such SDKs when necessary.

Use cases:

• Call some Flutter-specific API to support unbinding native dependencies on a Flutter hot-restart. When running in Dart-standalone this code does not need to be called. When running in Dart-standalone, such Flutter-specific API cannot be resolved in dart pub get. Add hot restart hooks to support dart:ffi flutter/flutter#75528
• Call the Flutter plugin API to get the Android Context on Android. This also requires a dependency on Android.