Re: [whatwg/streams] Test or specification problem with TransformStream readable.cancel and calling controller.error (Issue #1296)

@saschanaz The WebIDL specification (as currently written) requires this.

In step 13 of [invoke a callback function type](https://webidl.spec.whatwg.org/#invoke-a-callback-function), the call result is converted to an IDL value. This must be an IDL Promise, whose [conversion](https://webidl.spec.whatwg.org/#es-promise) is defined as:

> 1. Let _promiseCapability_ be [?](https://tc39.es/ecma262/#sec-returnifabrupt-shorthands) [NewPromiseCapability](https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-newpromisecapability)([`%Promise%`](https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-promise-constructor)).
> 2. Perform [?](https://tc39.es/ecma262/#sec-returnifabrupt-shorthands) [Call](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-call)(_promiseCapability_.\[[Resolve]], **undefined**, « _V_ »).
> 3. Return _promiseCapability_.

It would be nice if this could use [the `PromiseResolve` abstract op](https://webidl.spec.whatwg.org/#a-promise-resolved-with) instead, but then the "shape" of an IDL Promise type would change. Right now, it's a [Promise Capability Record](https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-promisecapability-records), with \[[Promise]], \[[Resolve]] and \[[Reject]] internal slots. But `PromiseResolve` only returns the promise... Maybe we could give it no-op resolve and reject functions, since they won't be used anyway?

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