我试图建立我的sass与gulp sass和我得到的错误：
以下任务未完成：build，clean您是否忘记发出异步完成信号？

gulp.task('clean', function() { return del.sync('dist'); });

 gulp.task('build', gulp.series('clean', 'imagemin', 'sass', 'js', function(done) {

var buildMain = gulp.src([
    'app/*.html',
    'app/.htaccess'
]).pipe(gulp.dest('dist'));

var buildCss = gulp.src([
    'app/css/main.min.css'
    ])
.pipe(gulp.dest('dist/css'));

var buildJs =  gulp.src([
    'app/js/scripts.min.js'
    ])
.pipe(gulp.dest('dist/js'));

var buildFonts = gulp.src([
    'app/fonts/**/*'
    ])
.pipe(gulp.dest('dist/fonts'));
done(); }));