gulp.task('sass', function () {
    return gulp.src('./app/styles/*.scss')
        .pipe($.sass({
            style: "expanded",
            precision: 10,
            loadPath: ["app/bower_components"]
        }))
        .pipe($.uncss({
            html: ['./dist/*.html']
        }))
        .pipe($.csso())
        .pipe($.cssbeautify())
        .pipe($.autoprefixer("last 1 version"))
        .pipe(gulp.dest("dist/styles"))
        .pipe($.size());
});

C:sitegulp-4>gulp sass
[17:32:20] Using gulpfile C:sitegulp-4gulpfile.js
[17:32:20] Starting 'sass'...
[17:32:23] all files 874 B
[17:32:23] Finished 'sass' after 2.89 s

C:sitegulp-4>