function createSelector(fn) {
    return fn;
  }
Currently createSelector is just the identity function. The long-term
goal is for it to be a way to create optimizable selectors using React's
unstable_observedBits Context API. The implementation of that
optimization strategy is currently still in development, but I want people
to start using createSelector now. Then, when it *does* get optimized, there
will be changes required from users.