__FlutterEngineFlushPendingTasksNow was added as a workaround for embedders to flush tasks on threads where the engine didn't own the message loop (or couldn't interact with the same as is the case on Android and Darwin). This API is hard to use and was a temporary hack. Remove the same and allow embedders to specify thread up front. This API has landed on the C++ side via the TaskRunner configuration passed to the Shell::Create call. This has to be wrapped up in the stable C API and exposed to custom embedders.