Can't import ES2015 modules

[simonhaenisch]

Stencil version:

@stencil/core@0.12.4

I'm submitting a ...
[x] bug report
[ ] feature request
[ ] support request

Current behavior:
I installed sindresorhus/ky (version 0.3.0) into my project (which is an ES2015 module), then imported it. Now the build fails because of an object spread in the source:

[ ERROR ]  Plugin Error
           Unexpected token (19:19) in <project-dir>/node_modules/ky/index.js

Expected behavior:
I want to be able to import ES2015 modules and get them transpiled to ES5 during build (I thought that was already possible with Stencil?).

Related code:

import ky from 'ky';

sindresorhus/ama#446