Implementation using a timer for browsers that do not support animation frames.

class AnimationSchedulerImplTimer extends AnimationSchedulerImpl {

 /**
  * The default time in milliseconds between frames. 60 fps == 16.67 ms.
  */
 static int DEFAULT_FRAME_DELAY = 16;

 /**
  * The minimum delay in milliseconds between frames. The minimum delay is
  * imposed to prevent freezing the UI.
  */
 static int MIN_FRAME_DELAY = 5;

 /**
  * The list of animations that are currently running.
  */
 List<AnimationHandleImplTimer> animationRequests = new List<AnimationHandleImplTimer>();

 /**
  * The timer that updates all animations.
  */
 Timer timer;

 AnimationSchedulerImplTimer() {
   timer = new Timer.get(updateAnimations);
 }

 AnimationHandle requestAnimationFrame(AnimationCallback callback, [dart_html.Element element = null]) {
   // Save the animation frame request.
   AnimationHandleImplTimer requestId = new AnimationHandleImplTimer(callback, this);
   animationRequests.add(requestId);

   // Start the timer if it isn't started.
   if (animationRequests.length == 1) {
     timer.schedule(DEFAULT_FRAME_DELAY);
   }

   // Return the request id.
   return requestId;
 }

 bool isNativelySupported() {
   return true;
 }

 void cancelAnimationFrame(AnimationHandle requestId) {
   // Remove the request from the list.
   int index = animationRequests.indexOf(requestId);
   if (index != -1) {
     animationRequests.removeAt(index);
   }

   // Stop the timer if there are no more requests.
   if (animationRequests.length == 0) {
     timer.cancel();
   }
 }

 /**
  * Iterate over all animations and update them.
  */
 void updateAnimations() {
   // Copy the animation requests to avoid concurrent modifications.
   List<AnimationHandleImplTimer> curAnimations = new List<AnimationHandleImplTimer>.from(animationRequests);

   // Iterate over the animation requests.
   Duration duration = new Duration();
   for (AnimationHandleImplTimer requestId in curAnimations) {
     // Remove the current request.
     int index = animationRequests.indexOf(requestId);
     if (index != -1) {
       animationRequests.removeAt(index);
     }

     // Execute the callback.
     //cduration.inMilliseconds); //getStartMillis());
     requestId.getCallback().execute(new DateTime.now().millisecondsSinceEpoch);
   }

   // Reschedule the timer if there are more animation requests.
   if (animationRequests.length > 0) {
     /*
      * In order to achieve as close to 60fps as possible, we calculate the new
      * delay based on the execution time of this method. The delay will be
      * less than 16ms, assuming this method takes more than 1ms to complete.
      */
     timer.schedule(dart_math.max(MIN_FRAME_DELAY, DEFAULT_FRAME_DELAY - duration.inMilliseconds)); //elapsedMillis()));
   }
 }
}