function isEndish(topLevelType) {
  return topLevelType === topLevelTypes.topMouseUp ||
         topLevelType === topLevelTypes.topTouchEnd ||
         topLevelType === topLevelTypes.topTouchCancel;
}
Standard/simple iteration through an event's collected dispatches, but stops
at the first dispatch execution returning true, and returns that id.

@return id of the first dispatch execution who's listener returns true, or
null if no listener returned true.