This is a wrapper around package:async_helper from the Dart SDK source repo at //pkg/async_helper that works in the environment of flutter_tester. This wrapper is needed to ensure that all tests run to completion before the process exits. This is accomplished by opening a ReceivePort for each test, which is only closed when the test finishes running.
This package is intended only for use in the flutter/engine repo by unit tests that run on flutter_tester. Even though the API resembles the API provided by package:test, it has all the same limitations that package:async_helper has.