Re: [whatwg/dom] Define a 'CancellationController' and 'CancellationSignal' interface. (#434)

domenic commented on this pull request.



> @@ -1419,6 +1419,103 @@ that gave folks all the wrong ideas. <a>Events</a> do not represent or cause act
 can only be used to influence an ongoing one.
 
 
+<h3 id=controlling-promises>Controlling {{Promise}}s</h3>
+
+Though {{Promise}} objects don't have any built-in cancellation mechanism, many {{Promise}}-vending
+APIs require cancellation semantics. {{PromiseController}} is meant to support these requirements by
+providing an {{PromiseController/abort()}} method that toggles the state of a corresponding
+{{PromiseSignal}} object. The API which wishes to support cancellation can accept such a
+{{PromiseSignal}}, and use its state to determine how (not) to proceed.
+
+Upon {{PromiseController/abort()}}, the relevant {{Promise}} will reject with an
+<dfn exception>AbortError</dfn> [=simple exception=].

I'm not sure if we have a middle ground between "use DOMException" and "write an ES-style spec". I guess the ES-style spec could be small though, basically just saying that it fulfills the [_NativeError_ Object Structure](https://tc39.github.io/ecma262/#sec-nativeerror-object-structure) with name X.

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