I somehow missed it. oops
It is already somewhat bloated.
I guess all DOM/js api helpers. JS already passed this once and it was considered bad idea to extend any standard api. Although main reason named was monkeypathcing conflicts which does not affect kotlin code.
Problem is JS not being single target has IE, Chrome, Firefox, Node, Safari implementations. kotlin-js includes some random org.w3c.* api s. For example you provide fetch api in stdlib-js, but do you provide actual polyfill (no. but somehow still eats ~200 lines in kotlin.js)? Is it usable from other js modules (no)? Actually it is hard to test since chrome/firefox already shipping it. But iOS will be broken. This means one cannot be sure if api is present even if it is provided in kotlin-stdlib-js and call to declared method could result in “fetch is undefined”, or on IE very informative “error error”
Btw having separate modules like kotlin-js-dom2-ie-api seems logical but will not work. See detect browser features vs versions internet debates from decade ago.
Also kotlin typesystem is not very flexible and in general it is hard to express js api in kotlin. Try doing it with something like _.debounce
GWT way of solving of all this problems was implementing wrapping everything and providing full platform. There is com.google.gwt.user.client.Element, not org.w3c.dom.Element. Also swing like widgets library, custom centralized event system. It worked very well in 2005 for IE5.5. Not so much in 2017. Compared to React/Vue/Angular this approach is just too much work for less result.
Typescript solved this by not being crossplatform and targetting JS only.
There where lot of attempts to this kind of problem. They all end up in two ways. Java way - provide paltform. C way - provide tools to make platform specific code bearable. Looks like kotlin tries to avoid both
Personally my crosstarget language of the dreams should have smallish stdlib to make DTO/algo code crossplatform and totally skip the rest. Kind of cpp but with GC and sane macro system.
In general as I said before my kotlin js expirience is rather limited to moment and most problems I have encountered are from trying to reuse validators. Hope this helps.