How do I get TestScheduler to tick in RxJs5?

I'm trying to write a sample unit test with Observable.interval in RxJs version 5. I'm running the following code, but my observable only fires once, not 20 times, as anticipated.

it('does its interval thing synchonously', ()=> {

        let x = [];
        let scheduler = new Rx.TestScheduler();
        let interval$ = Rx.Observable.interval(500, scheduler).take(20);

        interval$.subscribe(
            value => {
                x.push(value);
                console.log(value)
            },
        );

        for(let i = 0; i < 20; i++) {
            scheduler.flush();
        }

        expect(x.length).toBe(20);

    });

How do I make my TestScheduler move the Observable forward by 10000 milliseconds?