ES6ify BookReader.prototype.prefetch + make it prefetch only viewable pages
ES6ify BookReader.prototype.prefetch + make it prefetch only viewable pages

openlibrary/bookreader,internetarchive/bookreader,openlibrary/bookreader,internetarchive/bookreader,openlibrary/bookreader,openlibrary/bookreader,openlibrary/bookreader