Returns a Future that completes just before the window is about to repaint so the user can draw an animation frame.
If you need to later cancel this animation, use [requestAnimationFrame] instead.
The [Future] completes to a timestamp that represents a floating point value of the number of milliseconds that have elapsed since the page started to load (which is also the timestamp at this call to animationFrame).
Note: The code that runs when the future completes should call [animationFrame] again for the animation to continue.