code: function promiseChanges(object, types) {
    return new Promise(function(resolve) {
      function observer(changes) {
        resolve(changes);
        unobserve(object, observer);
      }
      observe(object, observer, types);
    });
  }
docs: Promise from one-time Object.observe. Usage: ```promiseChanges(obj, ['update']).then(function(changes) {...`