Loosen brand checks on ReadableStream methods

[MattiasBuelens]

In #96, #98 and #99, we made pipeTo(), pipeThrough(), tee() and [Symbol.asyncIterator]() use only the public API rather than relying on internal abstract operations.

This PR brings it all together by loosening the brand checks on these methods. You'll now be able to call readable.pipeTo(writable) with any writable that "looks like" a WritableStream, i.e. anything that has a sufficiently compliant getWriter() method. This includes native WritableStreams, or even WritableStreams constructed by a different polyfill. 😁

In theory, you could also call these on anything that "looks like" a ReadableStream, although it's currently still a bit awkward:

import { ReadableStream, WritableStream } from 'web-streams-polyfill';
const nativeReadable = new Response("Hello world!").body;
const destination = new WritableStream({ /* ... */ });
const pipePromise = ReadableStream.prototype.pipeTo.call(nativeReadable, destination);

[jimmywarting]

const pipePromise = ReadableStream.prototype.pipeTo.call(nativeReadable, destination);

Well, that's some hacky shit :P
would be better if something was extended or patched based on the native (builtin)

[MattiasBuelens]

would be better if something was extended or patched based on the native (builtin)

Yes, I'll get to that. This isn't a final solution yet. 😉