main.js import a css file throw a error(dependencies were not found)

[cutpictureboy]

import Vue from 'vue'
import App from './App'
import router from './router'
import ElementUI from 'element-ui'
import 'element-ui/lib/theme-chalk/index.css'

Vue.config.productionTip = false
Vue.use(ElementUI)

when i change the config in build/webpack.dev.config.js
old

const devWebpackConfig = merge(baseWebpackConfig, {
  module: {
    rules: utils.styleLoaders({ sourceMap: config.dev.cssSourceMap, usePostCSS: true })
  },

new

const devWebpackConfig = merge(baseWebpackConfig, {
  module: {
    rules: utils.styleLoaders({ sourceMap: config.dev.cssSourceMap, usePostCSS: false })
  },

it work well

[cutpictureboy]

today, i use vue init webpack initialize a project and not found this problem, but i use vue init webpackyesterday，this problems occur many times, so the problem may not exist