Re: [whatwg/streams] add @@asyncIterator to ReadableStream (#954)

MattiasBuelens requested changes on this pull request.

One more thing we missed in the spec text: `next()` and `return()` should return rejected promises instead of throwing. @domenic got it right in their proposed test plan.

> +  </thead>
+  <tr>
+    <td>\[[asyncIteratorReader]]
+    <td class="non-normative">A {{ReadableStreamDefaultReader}} instance
+  </tr>
+  <tr>
+    <td>\[[preventCancel]]
+    <td class="non-normative">A boolean value indicating if the stream will be <a lt="cancel a readable
+    stream">canceled</a> when the async iterator's {{ReadableStreamAsyncIteratorPrototype/return()}} method is called
+  </tr>
+</table>
+
+<h4 id="rs-asynciterator-prototype-next" method for="ReadableStreamAsyncIteratorPrototype">next()</h4>
+
+<emu-alg>
+  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, throw a *TypeError* exception.

This should return a rejected promise, rather than throw an exception. From [this comment](https://github.com/whatwg/streams/pull/954#issuecomment-421107013) by @domenic:
> Calling next() or return() on non-ReadableStreamDefaultReaderAsyncIterator instances rejects with TypeError (there are existing brand check tests you can add to)

> +</table>
+
+<h4 id="rs-asynciterator-prototype-next" method for="ReadableStreamAsyncIteratorPrototype">next()</h4>
+
+<emu-alg>
+  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, throw a *TypeError* exception.
+  1. Let _reader_ be *this*.[[asyncIteratorReader]].
+  1. If _reader_.[[ownerReadableStream]] is *undefined*, return <a>a promise rejected with</a> a *TypeError* exception.
+  1. Return ! ReadableStreamDefaultReaderRead(_reader_, *true*).
+</emu-alg>
+
+<h4 id="rs-asynciterator-prototype-return" method
+for="ReadableStreamAsyncIteratorPrototype">return( <var>value</var> )</h4>
+
+<emu-alg>
+  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, throw a *TypeError* exception.

Same as above, this should return a rejected promise.

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