code: function () {
      /**
       * Return the current timestamp integer.
       */
      var now = Date.now || function () {
        return new Date().getTime();
      };
  
      // Keep track of the previous "animation frame" manually.
      var prev = now();
  
      return function (fn) {
        var curr = now();
        var ms   = Math.max(0, 16 - (curr - prev));
        var req  = setTimeout(fn, ms);
  
        prev = curr;
  
        return req;
      };
    }
docs: Fallback animation frame implementation.
  
  @return {Function}