HTML_DocumentDependencies_Item is pretty much a lesser re-implementation of Net_URL. Please switch to using that package. If you absolutely must have more functionality, extend Net_URL.
Use HTTP_Request instead of fopen and file_get_contents. Some people don't have allow_url_fopen turned on. In addition, it allows for more fine-grained control of fetching and can deal with redirects and such.
_setupSourceProperties should use Net_URL.
Your source link seems to be broken, it ends in getAll(). Hopefully there isn't any more substantial code beneath there.