2018 Update: The module loader spec is now a part of the ES Spec – what you are describing is allowed and possible with <script type="module"> in browsers and with a custom --loader with Node.js as well as with Deno if you’re into that.
The module loader spec and the import/export syntax are separate. So this is a property of the module loader (not a part of the ES spec). If you use a module loader that supports plugins like SystemJS.