定义过了cross-env NODE_ENV=production，为什么还要在DefinePlugin里面定义一遍？
"scripts": {
"build": "cross-env NODE_ENV=production webpack --config webpack.config.js",
"dev": "cross-env NODE_ENV=development webpack-dev-server --config webpack.config.js"
},
plugins: [
new webpack.DefinePlugin({
'process.env': {
NODE_ENV: isDev ？ '"development"': '"production"'
}
})]
