Howto mock a Worker ?

[David Mouton]

Hi all,
I'm writing some unit test.
I have one class wich use an instance of a Worker

public function new( worker:Worker, playerId:String)
    {
        init();
        _playerId = playerId;
        _woker = worker;
        _woker.onmessage = worker_messageHandler;
    }

Usually, i use an interface like IWorker to use a mock instead of a Worker, but Worker has no interface.
So i tried to extends Worker with my mock, but there is a JS error at runtime, maybe beacuse this is an extern clas or a native things.

Any idea ?