- virtual files now have a .js extension by default
- systemjs module references include this file extension
- amd module references omit this file extension. this seems to be
  consistent with amd output of other chunks

Use entryFileNames to change the extension of the filename to be mjs,
and see that the file created from the virtual file chunk has a .mjs
extension

Note that the amd definition contains the .mjs extension, as amd ids only
strip the .js file extension, which is slightly odd but probably ok as
this amd is not the targetted usecase for this behaviour