code: function observePromise(proxy, promise) {
    promise.then(value => {
      set(proxy, 'isFulfilled', true);
      value._settingFromFirebase = true;
      set(proxy, 'content', value);
      value._settingFromFirebase = false;
    }, reason => {
      set(proxy, 'isRejected', true);
      set(proxy, 'reason', reason);
      // don't re-throw, as we are merely observing
    });
  }
docs: reimplemented private method from Ember, but with setting _settingFromFirebase so we can avoid extra saves down the line