code: @SuppressWarnings("unchecked")
    @GwtIncompatible("ObjectOutputStream")
    public void saveContents(ObjectOutputStream out) throws IOException {
      out.writeObject(eachRefTypeIndexedByProperty);
      out.writeObject(interfaceToImplementors);
      out.writeObject(typesIndexedByProperty);
    }
docs: Saves the derived state.
  
  Note: This should be only used when serializing the compiler state and needs to be done at the
  end, after serializing CompilerState.