code: function addSampleData() {
    return Promise.all([
      insertPersonData('actor.json', db.getSchema().table('Actor')),
      insertPersonData('director.json', db.getSchema().table('Director')),
      insertData('movie.json', db.getSchema().table('Movie')),
      insertData('movieactor.json', db.getSchema().table('MovieActor')),
      insertData('moviedirector.json', db.getSchema().table('MovieDirector')),
      insertData('moviegenre.json', db.getSchema().table('MovieGenre'))
    ]).then(function(queries) {
      var tx = db.createTransaction();
      return tx.exec(queries);
    });
  }
docs: Adds sample data to the database.
  @return {!IThenable}