Make pipeThough() non-generic?

[ricea]

Currently pipeThrough() operates generically on this. It performs no brand check, and will attempt to call this.pipeTo() regardless of whether this is a ReadableStream or not.

Blink is currently changing its streams implementation to use IDL, and there's no way to directly express this in IDL. We will need some workaround to pass the tests.

Having pipeThrough() be generic complicates the standard (because we have to check if the return value of pipeTo() is a Promise), and complicates IDL-based implementations. I think it doesn't provide much value, and the easiest thing to do would be to add a brand check and make it call an abstract PipeTo operation internally.

@domenic, what do you think?

[domenic]

I'm fine with this, because I've generally come to regret all these gratuitous differences with most web-based specs. See #963 for the larger discussion there.

If we do pursue #963, implementations may want to bundle this change in with the many other small tweaks that would be involved, so as to minimize any transition pain from web sites depending on the current semantics. But, we could also do this independently ahead of time.