code: protected boolean isViewBuilt(FacesContext facesContext, UIViewRoot view)
    {
        return Boolean.TRUE.equals(facesContext.getAttributes().get(view));
    }
docs: Returns true if the given UIViewRoot has already been built via VDL.buildView().
  This is necessary because legacy ViewHandler implementations return null on
  getViewDeclarationLanguage() and thus VDL.buildView() is never called.
  So we have to check this in renderView() and, if it is false, we have to
  call buildView() manually before the rendering.
  
  @param facesContext
  @param view
  @return