[dom] MutationObserver callbacks should be called before executing defer scritps

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

== MutationObserver callbacks should be called before executing defer 
scritps ==
(Cloned from https://bugzilla.mozilla.org/show_bug.cgi?id=1180927)

This doesn't seem to be spec'ed currently (I filled a W3C bug [0]) but
 if would be nice to follow the implementation of Chrome and Safari 
here and execute Mutation Observer callbacks before we execute 
deferred scripts.

That would allow us to prototype API's with shims that rely on 
Mutation Observer.


[0] https://www.w3.org/Bugs/Public/show_bug.cgi?id=28920

See https://github.com/whatwg/dom/issues/78