Question: dynamic import() seems to expect .js files, what about .mjs?

Hi!  

If this is a silly question, forgive me.  I would expect .mjs to be supported for the .import(*) handler, but maybe I'm misunderstanding the ES Modules proposal?  Or maybe this is not for thosetrying to use the `--experimental-modules` feature in Node?  I'm playing catch up as we move from Node 6 to Node 8 and trying to get off Babel.  Your insight is appreciated.

Thanks!  - Aaron