For ExtendableMessageEvent:
Once reference circulation between Blink and V8 is solved (http://crbug.com/501866), update the constructor to be:
Constructor(DOMString type, optional ExtendableMessageEventInit eventInitDict)
For CustomEvent:
See crbug.com/468240
For the other two:
It seems like they all have something to do with Callbacks.
Comment 1 by lunalu@chromium.org
, Mar 13 2017