Add [[PlatformBrand]] internal slot to all platform objects

[annevk]

The [[PlatformBrand]] internal slot would be a List (in the ECMAScript sense) of strings. One string per interface on the interface chain of the object. So e.g., Text would have "Text, "CharacterData", "Node", "EventTarget".

This would allow us to be more specific what we mean with instances of something and would also clear up a lot of language in IDL I think.

[dominiccooney]

Is the [[PlatfromBrand]] on the instance (for example a text node in the DOM, not the Text interface or Text interface prototype object?)

For what it is worth Blink implements the check for HTMLElement in custom elements as if the brand is on the instance; see CustomElementDefinition::checkConstructorResult. Sorry for the C++, but basically it checks the element instance is a HTMLElement; it doesn't consult JavaScript prototypes or anything.

[annevk]

Yes, on the instance. All internal slots are I think.

[annevk]

(Once we add this StructuredSerialize can be simplified a little bit as all platform objects will have this slot and therefore can be treated like promises or WeakMap objects.)

[annevk]

The other thing that would be useful here is some shorthands around it. When you check against the list of platform brands, there's a difference if the matched value is first in the list or somewhere else (or last and somewhere else, depending on order).

E.g., in DOM we sometimes want to match DocumentFragment, but not ShadowRoot (a subclass).