Currently, interfaces which are not non-callback interfaces but still have an interface object because they have constants don't have their own name. @bzbarsky suggested during a Mozlando meeting to call them "legacy callback interface objects". The "legacy" part comes from the fact that such interfaces should be avoided anyway.

We should decide whether such legacy callback interface objects (i.e. NodeFilter, the only one such interface like that in the wild) should have a [[HasInstance]] method, and the one for noncallback interface objects can be simplified: steps 2 and 3 are useless for the latter because their "prototype" property is non-configurable, and thus always an object.

Cc @heycam