I think your problem is, that you want to concat/minify the angular sources. The documented way is to use the provided files like angular-route.min.js and include them in the head section of your html.
So, following lines throws that error:
<script src="/app/bower_components/angular/angular.js"></script>
<!-- build:js /app/scripts/angular.js -->
<script src="/app/bower_components/angular-route/angular-route.js"></script>
<script src="/app/bower_components/angular-resource/angular-resource.js"></script>
<script src="/app/bower_components/angular-cookies/angular-cookies.js"></script>
<script src="/app/bower_components/angular-sanitize/angular-sanitize.js"></script>
<!-- endbuild -->
As a side note: these angular libs heave heavy complex sourcecode in some parts, its quite a challenge to exactly configure your personal minifier, like the angular developers did with theirs, to provide the public angular-xxxxx.min.js files.