We have 2 blocks defined in our index.html - one for 3rd party libs and one for our application files. Since 3rd party libs are already minified, we just want to concatenate them, but not uglify. How can I do this with useminPrepare?
<!-- build:js js/lib.js -->
<script src="lib/angular/angular.min.js"></script>
<script src="lib/angular-cookies/angular-cookies.min.js"></script>
<script src="lib/angular-route/angular-route.min.js"></script>
<!-- endbuild -->
 <!-- build:js js/app.js -->
<script src="js/app.js"></script>
<script src="js/controllers/LanguageCtrl.js"></script>
<!-- endbuild -->
gruntfile.js:
useminPrepare: {
        html: '<%= yeoman.app %>/index.html',
        options: {
            dest: '<%= yeoman.dist %>',
            flow: {
                html: {
                    steps: {
                        // TODO for libs.js block I don't want uglify!
                        js: ['concat', 'uglifyjs'], 
                        css: ['cssmin']
                    },
                    post: {}
                }
            }
        }
    }