[css-typed-om-1]The IDL for StylePropertyMap is not valid due to multiple maplike declarations

[bzbarsky]

https://heycam.github.io/webidl/#idl-maplike clearly says:

The inherited interfaces of a maplike interface must not also have a maplike declaration.

but that's precisely what StylePropertyMap is doing: inheriting from another interface with a maplike declaration.

The reason for this restriction is that a maplike declaration induces the existence of a [[BackingMap]] internal slot on instances. When there are multiple maplike declarations it's undefined whether there are multiple internal slots or whether they interact with the same internal slot or something else.

[tabatkins]

(Moot now that I'm not using maplike<>, but...)

Hm, I was specifically using readonly maplike<> on the (readonly) superclass on maplike<> on the (readwrite) subclass; I figured that was how it was supposed to be done, from the similar examples elsewhere in WebIDL.