nsIAsync(In|Out)putStream are xpcom components and can be invoked in JS with Services.tm.currentThread specified as the nsIEventTarget of the ns(In|Out)putStreamReadyEvent runnable in http://searchfox.org/mozilla-central/search?q=asyncWait&case=true&regexp=false&path= However, the EventTarget of DocGroup/TabGroup/SystemGroup are not available in JS level.

:billm, do we have any plan to have labeling API in JS level that will be used in comment 0?

No, we don't have anything like that. I think we could add a getter for the SystemGroup to the thread manager. If we need access to the DocGroup or TabGroup, we could maybe add a method to nsIDOMWindowUtils.

I'd like to lower the priority of this bug to P3 because the use of asyncWait are in dev/tool and the old add-on sdk and dom/presentation(which is only enabled in Android) according to search result in comment 0. None of these will contribute the high frequency of nsInputStreamReadyEvent.