normalizeFrames = function normalizeFrames(
  source: Animation,
  cache: FrameMap
): Animation {
  const normalized: Animation = {};
  for (const frame in source) {
    const normalizedFrame = defaultNormalizedFrames[frame] || `${Math.round(
      parseFloat(frame) / keyframeDistance
    ) * keyframeDistance }%`;
    normalized[normalizedFrame] = source[frame];
    cache[normalizedFrame] = normalizedFrame;
  }
  return normalized;
}
Takes a source animation and the current cache, populating the
cache with the normalized keyframes and returning a copy of the
source animation with the normalized keyframes as well.

It uses keyframeDistance to determine how much it should normalize
frames.
@param {Object} source Animation
@param {Object} cache FrameMap
@returns {Object} Animation
@private