Hi,
I am using plugin-sass, which uses sass.js for parsing.
I logged dougludlow/plugin-sass#74, but the authors seem to think the problem is upstream.

The problem is that I cannot seem to import files that contain @charset "UTF-8";.

If you are interested, I have created a test repository to illustrate the problem: https://github.com/tjaartvdwalt/systemjs-sass-plugin-example