[dom] DOM should add hooks for sync events that happen during insert/remove operations

travisleithead has just created a new issue for 
https://github.com/whatwg/dom:

== DOM should add hooks for sync events that happen during 
insert/remove operations ==
As discsussed at Custom Elements F2F:
* blur
* focus
* beforeunload
(other?)
...events require some synchronous processing during tree insertion 
and/or removal
(This may have been discussed during the part of the meeting where 
there was nothing scribed...?)

See https://github.com/whatwg/dom/issues/57