Register classes with setOldClass

[nuest]

Issue Description and Expected Result

It is not possible to use the class xml_document or xml_node in formal definitions of S4 classes.
It would be helpful for packages using S4 if they could properly define slots as being of a type provided by this package.

This should be possible with setOldClass.

(Let me know if you'd welcome a PR on this.)

Reproducible Example

library(xml2)

setClass("Testclass",
         representation(xml = "xml_document"),
         prototype = list(xml = xml2::xml_missing()),
         validity = function(object) {
           #...
           return(TRUE)
         }
)

Error in makePrototypeFromClassDef(properties, ClassDef, immediate, where) : 
  in making the prototype for class “Testclass” elements of the prototype failed to match the corresponding slot class: xml (class "xml_document" )
In addition: Warning message:
undefined slot classes in definition of "Testclass": xml(class "xml_document") 

Session Info

devtools::session_info()

─ Session info ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 setting  value                       
 version  R version 3.5.2 (2018-12-20)
 os       Ubuntu 18.04.2 LTS          
 system   x86_64, linux-gnu           
 ui       RStudio                     
 language en_GB:en                    
 collate  en_GB.UTF-8                 
 ctype    en_GB.UTF-8                 
 tz       Europe/Berlin               
 date     2019-03-07                  

─ Packages ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 package     * version    date       lib source        
 assertthat    0.2.0      2017-04-11 [1] CRAN (R 3.5.1)
 backports     1.1.3      2018-12-14 [1] CRAN (R 3.5.1)
 bitops      * 1.0-6      2013-08-17 [1] CRAN (R 3.5.1)
 callr         3.1.1      2018-12-21 [1] CRAN (R 3.5.1)
 cli           1.0.1      2018-09-25 [1] CRAN (R 3.5.1)
 crayon        1.3.4      2017-09-16 [1] CRAN (R 3.5.1)
 desc          1.2.0      2018-05-01 [1] CRAN (R 3.5.1)
 devtools      2.0.1      2018-10-26 [1] CRAN (R 3.5.1)
 digest        0.6.18     2018-10-10 [1] CRAN (R 3.5.1)
 fs            1.2.6      2018-08-23 [1] CRAN (R 3.5.1)
 glue          1.3.0      2018-07-17 [1] CRAN (R 3.5.1)
 httr        * 1.4.0      2018-12-11 [1] CRAN (R 3.5.2)
 lattice       0.20-38    2018-11-04 [1] CRAN (R 3.5.1)
 magrittr      1.5        2014-11-22 [1] CRAN (R 3.5.1)
 memoise       1.1.0      2017-04-21 [1] CRAN (R 3.5.1)
 pkgbuild      1.0.2      2018-10-16 [1] CRAN (R 3.5.1)
 pkgload       1.0.2      2018-10-29 [1] CRAN (R 3.5.1)
 prettyunits   1.0.2      2015-07-13 [1] CRAN (R 3.5.1)
 processx      3.2.1      2018-12-05 [1] CRAN (R 3.5.1)
 ps            1.3.0      2018-12-21 [1] CRAN (R 3.5.1)
 R6            2.4.0      2019-02-14 [1] CRAN (R 3.5.2)
 Rcpp          1.0.0      2018-11-07 [1] CRAN (R 3.5.2)
 RCurl       * 1.95-4.11  2018-07-15 [1] CRAN (R 3.5.1)
 remotes       2.0.2      2018-10-30 [1] CRAN (R 3.5.1)
 rlang         0.3.1      2019-01-08 [1] CRAN (R 3.5.2)
 rprojroot     1.3-2      2018-01-03 [1] CRAN (R 3.5.1)
 rstudioapi    0.9.0      2019-01-09 [1] CRAN (R 3.5.2)
 sessioninfo   1.1.1      2018-11-05 [1] CRAN (R 3.5.1)
 sos4R       * 0.3.0.9002 2019-03-07 [1] local         
 sp          * 1.3-1      2018-06-05 [1] CRAN (R 3.5.1)
 stringi       1.3.1      2019-02-13 [1] CRAN (R 3.5.2)
 stringr     * 1.4.0      2019-02-10 [1] CRAN (R 3.5.2)
 testthat    * 2.0.1      2018-10-13 [1] CRAN (R 3.5.1)
 usethis       1.4.0      2018-08-14 [1] CRAN (R 3.5.1)
 withr         2.1.2      2018-03-15 [1] CRAN (R 3.5.1)
 XML           3.98-1.17  2019-02-08 [1] CRAN (R 3.5.2)
 xml2        * 1.2.0      2018-01-24 [1] CRAN (R 3.5.1)
 yaml          2.2.0      2018-07-25 [1] CRAN (R 3.5.1)

[1] /home/daniel/R/x86_64-pc-linux-gnu-library/3.5
[2] /usr/local/lib/R/site-library
[3] /usr/lib/R/site-library
[4] /usr/lib/R/library