Overloads of asyncAppend(batch, element), asyncAppend(element) makes optional batch awkward

[esprehn]

ex. a library will need to null check the batch and the arguments which is weird.

append(element, batch) means a library like:

doSomething(optional batch = null)

could exist without using apply() and prepending the batch if it's not null.

Can we spec both rest arguments for the start followed by batch?

ex.

asyncAppend(Node... node, DOMBatch = null)

not sure what webidl allows.

@tabatkins @domenic

[tabatkins]

I'm pretty sure WebIDL can't do rest arguments followed by additional arguments. I know JS can't; you have to just do a normal rest arg and then pull off the end manually. So we'd have to handle this in prose for all the methods that have rest args (are there actually any?).