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To create a unit test for asynchronous code using Dart Test, you can follow these steps:

1. Import the necessary libraries:

import 'package:test/test.dart';
import 'dart:async';

1. Write the test case with an async function:

void main() {
  test('test asynchronous code', () async {
    // Write your test code here
  });
}

1. Write your asynchronous code inside the test function. You can use the Completer class to create a future that completes asynchronously:

void main() {
  test('test asynchronous code', () async {
    Completer completer = new Completer();
    // Use the Timer class to simulate asynchronous code that completes in 2 seconds
    Timer(Duration(seconds: 2), () {
      completer.complete();
    });
    // Use the await keyword to wait for the future to complete
    await completer.future;
    // Write your expectations here
    expect(1+1, equals(2));
  });
}

1. Write your assertions using the expect function, as shown above.

Note: Asynchronous unit tests in Dart Test should always begin with "test" to be executed.