There isn't anything special about this, we can specify it in IDL and autogenerate some of it.
Created attachment 134204 [details] [PATCH] Proposed Fix
Created attachment 134205 [details] [GENERATED FILE] WebKit.framework/Versions/A/PrivateHeaders/DOMNodePrivate.h This is what the generated file looks like and ends up. Its implementation is in DOMNode.mm as expected.
Comment on attachment 134204 [details] [PATCH] Proposed Fix Clearing flags on attachment: 134204 Committed r112424: <http://trac.webkit.org/changeset/112424>
All reviewed patches have been landed. Closing bug.