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

MattiasBuelens requested changes on this pull request.

Still some holes left in `return()`.

> +</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.
+  1. Let _reader_ be *this*.[[asyncIteratorReader]].
+  1. If *this*.[[preventCancel]] is *false*, then:
+    1. If _reader_.[[ownerReadableStream]] is *undefined*, return <a>a promise rejected with</a> a *TypeError*
+    exception.
+    1. Let _result_ be ! ReadableStreamReaderGenericCancel(_reader_, _value_).
+    1. Perform ! ReadableStreamReaderGenericRelease(_reader_).
+    1. Return the result of <a>transforming</a> _result_ by a fulfillment handler that returns !
+    ReadableStreamCreateReadResult(_value_, *true*, *true*).
+  1. Perform ! ReadableStreamReaderGenericRelease(_reader_).

This should reject if `reader.[[ownerReadableStream]]` is `undefined`. Otherwise, we may fail an assert in `ReadableStreamReaderGenericRelease`.

We should probably just move step 3a to before step 3.

> +</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.
+  1. Let _reader_ be *this*.[[asyncIteratorReader]].
+  1. If *this*.[[preventCancel]] is *false*, then:
+    1. If _reader_.[[ownerReadableStream]] is *undefined*, return <a>a promise rejected with</a> a *TypeError*
+    exception.
+    1. Let _result_ be ! ReadableStreamReaderGenericCancel(_reader_, _value_).
+    1. Perform ! ReadableStreamReaderGenericRelease(_reader_).
+    1. Return the result of <a>transforming</a> _result_ by a fulfillment handler that returns !
+    ReadableStreamCreateReadResult(_value_, *true*, *true*).
+  1. Perform ! ReadableStreamReaderGenericRelease(_reader_).

What happens when there are pending read requests? [See previous discussion.](https://github.com/whatwg/streams/pull/950#issuecomment-417393457)

Normally, `ReadableStreamDefaultReader.releaseLock()` checks whether `this.[[readRequests]]` is empty. However, this implementation uses `ReadableStreamReaderGenericRelease` directly without going through the same checks as `releaseLock`.

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