Re: [w3ctag/design-reviews] HTML General Review: Cross-Document Messaging (#269)

The [lack of an `onclose` event in the `MessageChannel` interface](https://html.spec.whatwg.org/multipage/web-messaging.html#channel-messaging) presents a persistent usability hazard.

Previously offered arguments about GC observability have not withstood the passage of time. The addition of Web Audio Worklets, [Shared Array Buffers](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer), and [Atomic](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics). Recently renewed work on [Weak References](https://github.com/tc39/proposal-weakrefs/blob/master/specs/Weak%20References%20for%20EcmaScript.pdf) highlights the ways in which observability across turns is not a threat. Read consistency is only required within the same turn, and it is easy to satisfy this requirement by adding one-turn latency in dispatching close notifications. This should have no adverse performance impact.

Please add port closed events! 

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