Re: [whatwg/fetch] Task destination can stay null and then lead to undefined behavior (Issue #1673)

> Should there be an assertion that callers set either a client or useParallelQueue?

I believe this is the intent. Indeed, maybe _useParallelQueue_ should not be an explicit argument, and setting client to null should automatically cause the parallel queue to be used?

https://fetch.spec.whatwg.org/#fetch-elsewhere-request discusses this a bit:

> Set your [request](https://fetch.spec.whatwg.org/#concept-request)’s [client](https://fetch.spec.whatwg.org/#concept-request-client) to the [environment settings object](https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object) you’re operating in. Web-exposed APIs are generally defined with Web IDL, for which every object that implements an [interface](https://webidl.spec.whatwg.org/#dfn-interface) has a [relevant settings object](https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object) you can use. For example, a [request](https://fetch.spec.whatwg.org/#concept-request) associated with an [element](https://dom.spec.whatwg.org/#concept-element) would set the [request](https://fetch.spec.whatwg.org/#concept-request)’s [client](https://fetch.spec.whatwg.org/#concept-request-client) to the element’s [node document](https://dom.spec.whatwg.org/#concept-node-document)’s [relevant settings object](https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object). All features that are directly web-exposed by JavaScript, HTML, CSS, or other [Document](https://dom.spec.whatwg.org/#document) subresources should have a [client](https://fetch.spec.whatwg.org/#concept-request-client).
>
> If your [fetching](https://fetch.spec.whatwg.org/#concept-fetch) is not directly web-exposed, e.g., it is sent in the background without relying on a current [Window](https://html.spec.whatwg.org/multipage/nav-history-apis.html#window) or [Worker](https://html.spec.whatwg.org/multipage/workers.html#worker), leave [request](https://fetch.spec.whatwg.org/#concept-request)’s [client](https://fetch.spec.whatwg.org/#concept-request-client) as null and set the [request](https://fetch.spec.whatwg.org/#concept-request)’s [origin](https://fetch.spec.whatwg.org/#concept-request-origin), [policy container](https://fetch.spec.whatwg.org/#concept-request-policy-container), [service-workers mode](https://fetch.spec.whatwg.org/#request-service-workers-mode), and [referrer](https://fetch.spec.whatwg.org/#concept-request-referrer) to appropriate values instead, e.g., by copying them from the [environment settings object](https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object) ahead of time. In these more advanced cases, make sure the details of how your fetch handles Content Security Policy and [referrer policy](https://w3c.github.io/webappsec-referrer-policy/#referrer-policy) are fleshed out. Also make sure you handle concurrency, as callbacks (see [Invoking fetch and processing responses](https://fetch.spec.whatwg.org/#fetch-elsewhere-fetch)) would be posted on a [parallel queue](https://html.spec.whatwg.org/multipage/infrastructure.html#parallel-queue). [[REFERRER]](https://fetch.spec.whatwg.org/#biblio-referrer) [[CSP]](https://fetch.spec.whatwg.org/#biblio-csp)
>
> [...]
>
> Note that the [fetch](https://fetch.spec.whatwg.org/#concept-fetch) operation starts in the same thread from which it was called, and then breaks off to run its internal operations [in parallel](https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel). The aforementioned callbacks are posted to a given [event loop](https://html.spec.whatwg.org/multipage/webappapis.html#event-loop) which is, by default, the [client](https://fetch.spec.whatwg.org/#concept-request-client)’s [global object](https://html.spec.whatwg.org/multipage/webappapis.html#concept-settings-object-global). To process responses [in parallel](https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel) and handle interactions with the main thread by yourself, [fetch](https://fetch.spec.whatwg.org/#concept-fetch) with [useParallelQueue](https://fetch.spec.whatwg.org/#fetch-useparallelqueue) set to true.

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