BaseTestEventSinkType
Base expect type for TestEventSink. This layer of indirection is necessary because Kotlin/JS does not allow extension of function types. To work around it, we make this interface extend (UiEvent) -> Unit on all platforms except JS, and then expose an asEventSinkFunction() extension function on TestEventSink in JS that returns a function wrapper around it.
This type is not intended to be used directly or implemented by consumers of this library.