Using Sass toolkit with Ember

I'm trying to use this Sass toolkit with Ember. I have Sass installed using Ember-cli-sass

I have a working app.scss file that's being imported.

bower install sass-toolkit --save-dev

However when I then try to include any of the mixins they're not found:

@include clearfix('false');

results in :

The Broccoli Plugin: [SassCompiler] failed with: Error: no mixin named clearfix