Web IDL maplike: Allow spec prose to specify how key-type should be compared?

http://heycam.github.io/webidl/#es-maplike defines the behavior of maplike
objects it in terms of delegation to an actual ES Map object. This means
that if the key-type is an object, keys are compared by object ID rather
than content. Thus, has() and get() will only return positive results if
the same object that is stored is passed.

This works for things like a Node, which the application could actually
obtain, but it doesn't work for values that are represented by objects.
Specifically, a value in a BufferSource. Should specifications be allowed
to specify a different behavior in prose or should we force them to
manually define the map methods? Should such non-compliant manual maps be
discouraged?


As a concrete example, for EME, we attempted to use a readonly maplike with
a key-type of BufferSource, which contains a binary key ID. This can be
made to work by comparing the value in the BufferSource, but this is not
compliant with the maplike spec. (See the EME spec bug
https://github.com/w3c/encrypted-media/issues/25 for more details.) If
there is no change to the maplike spec to allow prose to modify the
comparison behavior, EME will likely switch to a manually defined map-like
interface. (We could also use a primitive type to represent the key ID, but
that is inconsistent with other representations of binary data in the spec.)


David