[BIG BUG]fallback

[MartinsYong]

Just check this test case:

describe('rAF', () => {
    it('tick', done => {
        let start = new Date().getTime();
        let times = 0;
    
        raf(function tick () {
            if (++times === 10) {
                const interval = (new Date().getTime() - start);
                console.assert(interval >= 9 * 16, `should take at least ${times - 1} frames worth of wall time: ${interval}ms`);
                done();
            } else {
                raf(tick);
            }
        });
    })
});