The Leaflet JS mapping library has lots of plugins available. The Leaflet package for R provides direct support for some, but far from all, of these plugins, by providing R functions for invoking the plugins.
If you as an R user find yourself wanting to use a Leaflet plugin that isn't directly supported in the R package, you can use the technique shown here to load the plugin yourself and invoke it using JS code.
Problem solved: I have just had to reboot R after uninstalling the old version of htmlwidgets and then reinstall the 0.8 version with the fixed 237 issue.
Sorry for the trouble
Gaël