A legacy platform object is a non-global platform object that
implements a special operation. A special operation is a getter, setter
and/or deleter. This is particularly used for old collection types,
such as HTMLCollection, NodeList, etc.
This will be used to make these spec-compliant and remove their custom
wrappers. Additionally, it will be used to implement collections that
we don't have yet, such as DOMStringMap.
