unit.js

# karma config file

module.exports = function(config) {
  config.set({
    basePath: __dirname + '../../../',
    frameworks: ['jasmine'],
    browsers: ['PhantomJS'],
    reporters: ['dots'],
    autoWatch: false,

    singleRun: true,
    colors: true,

    files: [
      '../app/assets/javascripts/list.js.coffee',
      '../spec/javascripts/unit/*_spec.js.coffee'
    ],

    preprocessors: {
      '**/*.coffee': 'coffee'
    }

  });
}

# list.js.coffe
class List
  name: 'A list'

# Results in:
#
# $ karma start spec/karma/config/unit.js
# INFO [karma]: Karma v0.10.2 server started at http://localhost:9876/
# INFO [launcher]: Starting browser PhantomJS
# INFO [PhantomJS 1.9.2 (Mac OS X)]: Connected on socket OLwWJSrAgtc1o-u0csd6
# PhantomJS 1.9.2 (Mac OS X) ERROR
#         SyntaxError: Parse error
#         at /Users/<user>/code/<project>/app/assets/javascripts/list.js.coffee:1
# PhantomJS 1.9.2 (Mac OS X): Executed 0 of 0 ERROR (0.026 secs / 0 secs)

Maybe this?

preprocessors: {
'../*/.coffee': 'coffee'
}