Define internal slots as attributes

[garykac]

From Tab:
A reasonably common practice, which Bikeshed explicitly deals with, is to define this as an attribute, but with [[foo]] naming - double square brackets. This groups it with the IDL definitions in indexes, and you can ref it with the IDL linking syntax, like {{[[foo]]}}.

This also explicitly associates the private slot with the interface, which is exactly what it's actually doing.