title: About gulp task dependences
body: <p>I write <code>gulpfile.js</code> like:</p>
  
  <pre><code>...
  
  gulp.task('styles',function(){
    gulp.src('./src/sass/*.scss')
      .pipe(sass())
      .pipe(autoprefixer('last 2 version'))
      .pipe(concat('tmp.css'))
      .pipe(gulp.dest('./build/css'))
      .pipe(notify('Styles task complete'))
  })
  
  gulp.task('css',['styles'],function(cb){
    del(['./build/css/tmp.css'],cb)
  })
  .....
  </code></pre>
  
  <p>When I run <code>gulp css</code>,the <code>styles</code> task will first be execute, so  <code>./build/css/tmp.css</code> will be removed.</p>
  
  <p>But the <code>./build/css/tmp.css</code> is still exist.That is mean,when the <code>css</code> task be executed,the <code>styles</code> task has not yet be execute.</p>
  
  <p>Why the dependences not work? and how can I make sure the <code>'./build/css/tmp.css'</code> has exist before the <code>css</code> task be executed?</p>