javascript - gulp插件gulp-minify-css, 输出的all.min.css未压缩

gulpfile.js如下

输出的all.min.css是没有压缩的. 为什么!!!

var gulp = require('gulp'),                     //基础库
    imagemin = require('gulp-imagemin'),        //图片压缩
    minifycss = require('gulp-minify-css'),     //css压缩
    jshint = require('gulp-jshint'),            //js检查
    uglify = require('gulp-uglify'),            //js压缩
    rename = require('gulp-rename'),            //重命名
    concat = require('gulp-concat'),            //合并文件
    connect = require('gulp-connect'),          //web服务
    useref = require('gulp-useref'),            //处理引用
    del = require('del');                       //清空文件夹


var Path = {
    css_src: './src/css/*.css',
    css_dist: './dist/css/',
    css_theme_src: './src/css/themes/**/*.*',
    css_theme_dist: './dist/css/themes/',
    img_src: './src/image/*.*',
    img_dist: './dist/image/',
    js_src: './src/js/**/*.js',
    js_dist: './dist/js/',
    js_lib_src: './src/lib/*.js',
    js_lib_dist: './dist/lib/',
    html_src: './src/**/*.html',
    html_dist: './dist/'
};


// 样式处理
gulp.task('css', function () {
    gulp.src(Path.css_src)
        .pipe(concat('all.min.css'))
        .pipe(minifycss())
        .pipe(gulp.dest(Path.css_dist))
        .pipe(connect.reload());

    gulp.src(Path.css_theme_src)
        .pipe(gulp.dest(Path.css_theme_dist))
        .pipe(connect.reload());
});


// 图片处理
gulp.task('image', function () {
    gulp.src(Path.img_src)
        .pipe(imagemin())
        .pipe(gulp.dest(Path.img_dist))
        .pipe(connect.reload());
});


// js处理
gulp.task('js', function () {
    gulp.src(Path.js_lib_src)
        .pipe(rename({suffix: '.min'}))
        .pipe(uglify())
        .pipe(gulp.dest(Path.js_lib_dist))
        .pipe(connect.reload());

    gulp.src(Path.js_src)
        .pipe(jshint('.jshintrc'))
        .pipe(jshint.reporter('default'))

        .pipe(concat('all.min.js'))
        .pipe(uglify())
        .pipe(gulp.dest(Path.js_dist))
        .pipe(connect.reload());
});


// HTML处理
gulp.task('html', function () {
    gulp.src(Path.html_src)
        .pipe(useref())
        .pipe(gulp.dest(Path.html_dist))
        .pipe(connect.reload());
});


// 清空img css js html
gulp.task('clean', function () {
    del.sync(['./dist/css', './dist/image', './dist/js', './dist/**/*.html'])
});


//服务
gulp.task('connect', function () {
    connect.server({
        livereload: true
    });
});


// 默认任务
gulp.task('default', ['clean', 'watch', 'connect'], function () {
    gulp.start('css', 'image', 'js', 'html');
});


// 监听任务 运行语句 gulp watch
gulp.task('watch', function () {

    // 监听css
    gulp.watch('./src/css/*.css', function () {
        gulp.run('css');
    });

    // 监听image
    gulp.watch('./src/image/*.*', function () {
        gulp.run('image');
    });

    // 监听js
    gulp.watch('./src/js/**/*.js', function () {
        gulp.run('js');
    });

    // 监听html
    gulp.watch('./src/**/*.html', function () {
        gulp.run('html');
    })
});