@Eric
I think a big question that you asked is if the module loader can load non js, I think that is a very important question.
Another problem is that in your example you have
import { Polymer } from 'bower_components/Polymer';
which could also be written as
<link rel="import" href="bower_components/Polymer/polymer.html">
and this is where the confusion comes, why two different ways to load assets, what I would like to have would be one way to load all assets. Html imports are ok, but if the ES6 module loader can load and manage all assets then wouldn't it be better to not use Html Imports. Html Imports is simple so maybe we keep it for simple cases, but the module loader does not need to be complicated either.
I would rather see a unified standard for loading assets implemented across all browsers and was hoping ES6 module loader might be that solution maybe it will evolve to that.
When it comes to Polymer if we use html imports which brings in our js files as well, then I guess we don't need to use module loader, but I can see this getting very ugly if an app uses part module loader part html imports. Also the de-duping feature is not very robust, just having two jquery files from two different cdn's in two different web components defeats the de-dup feature.