function timestampForTouch(touch) {
  // The legacy internal implementation provides "timeStamp", which has been
  // renamed to "timestamp". Let both work for now while we iron it out
  // TODO (evv): rename timeStamp to timestamp in internal code
  return touch.timeStamp || touch.timestamp;
}
Tracks the position and time of each active touch by `touch.identifier`. We
should typically only see IDs in the range of 1-20 because IDs get recycled
when touches end and start again.