code: function create(targetScrollY) {
    var scrollPositionStream = ScrollStream.create();
    var overTheLineStream = Stream.create();
    var pastScrollY = false;
    var firstRun = true;
  
    Stream.onValue(scrollPositionStream, function onScrollTrackPosition_(currentScrollY) {
      if ((firstRun || pastScrollY) && (currentScrollY < targetScrollY)) {
        pastScrollY = false;
        Stream.emit(overTheLineStream, ['before', targetScrollY]);
      } else if ((firstRun || !pastScrollY) && (currentScrollY >= targetScrollY)) {
        pastScrollY = true;
        Stream.emit(overTheLineStream, ['after', targetScrollY]);
      }
  
      firstRun = false;
    });
  
    return overTheLineStream;
  }
docs: Create a new Stream containing events which fire when a position has
  been scrolled past.
  @param {number} targetScrollY The position we are tracking.
  @return {Stream} The resulting stream.