You don't really need to assert implementation detail like this line and the line above. However you do need to assert the `idleCallback` function passed is called when the API execute the `onidle()` function.

I think the above only hold true if the time interval passes 100ms, so you might need to simulate that by modifying the number returned by `stubDateNow`. Actually, you need a mock but a stub here:

    Date.now = function() {
       return 1400000000000;
    };

You can pretend the time has passed by execute `setTimeoutStub.getCall(0).args[0]` too.