A couple of AsyncStub classes found here

public class ClassWithHTTPServiceDependencyTest extends TestCase
{
   private var _classBeingTested : ClassWithHTTPServiceDependency;
   private var _stub : HTTPServiceStub;
 
   override protected function setUp() : void
   {
      _classBeingTested = new ClassWithHTTPServiceDependency();
      _stub = new HTTPServiceStub("http://thisisntarealdomain.com");
      _stub.delay = 500;
      _classBeingTested.service = _stub;
   }
 
   public function testSendWithTokenResult() : void
   {
      var result : Function = function (event : DynamicEvent, passThroughData : Object) : void
      {
         assertEquals("GOAL!", event.payload);
      };
 
      _stub.result(null, "GOAL!");
 
      _classBeingTested.addEventListener("success", asyncHandler(result, 2000));
      _classBeingTested.someMethodUsingHTTPService();
   }
}