title: Block-scoped declarations not yet supported outside strict mode
body: <p>I'm running <strong>Laravel 5.4</strong> on my Homestead vagrant box. I've installed all the npm dependencies with <code>npm install</code> command. That didn't produce any errors.</p>
  
  <p>In my <strong>webpack.min.js</strong> file I have:</p>
  
  <pre><code>const { mix } = require('laravel-mix');
  
  /*
   |--------------------------------------------------------------------------
   | Mix Asset Management
   |--------------------------------------------------------------------------
   |
   | Mix provides a clean, fluent API for defining some Webpack build steps
   | for your Laravel application. By default, we are compiling the Sass
   | file for the application as well as bundling up all the JS files.
   |
   */
  
  mix.js([
          'resources/assets/plugins/jquery-1.11.3.min.js',
          'resources/assets/plugins/bootstrap/js/bootstrap.min.js',
          'resources/assets/js/main.js'
      ], 'public/js'
  );
  
  mix.combine([
         'resources/assets/plugins/bootstrap/css/bootstrap.min.css',
         'resources/assets/plugins/font-awesome/css/font-awesome.css',
         'resources/assets/css/styles.css'
     ], 'public/css/all.css');
  </code></pre>
  
  <p>When I want to run <code>npm run production</code> I'm getting the following errors:</p>
  
  <pre><code>&gt; @ production /home/vagrant/projects/nielsvroman
  &gt; node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js
  
  /home/vagrant/projects/nielsvroman/node_modules/laravel-mix/setup/webpack.config.js:120
          let extractPlugin = new plugins.ExtractTextPlugin(
          ^^^
  
  SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
      at exports.runInThisContext (vm.js:53:16)
      at Module._compile (module.js:404:25)
      at Object.Module._extensions..js (module.js:432:10)
      at Module.load (module.js:356:32)
      at Function.Module._load (module.js:311:12)
      at Module.require (module.js:366:17)
      at require (module.js:385:17)
      at requireConfig (/home/vagrant/projects/nielsvroman/node_modules/webpack/bin/convert-argv.js:96:18)
      at /home/vagrant/projects/nielsvroman/node_modules/webpack/bin/convert-argv.js:109:17
      at Array.forEach (native)
  
  npm ERR! Linux 3.19.0-25-generic
  npm ERR! argv "/usr/bin/nodejs" "/usr/bin/npm" "run" "production"
  npm ERR! node v5.0.0
  npm ERR! npm  v3.3.6
  npm ERR! code ELIFECYCLE
  npm ERR! @ production: `node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js`
  npm ERR! Exit status 1
  npm ERR!
  npm ERR! Failed at the @ production script 'node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js'.
  npm ERR! This is most likely a problem with the  package,
  npm ERR! not with npm itself.
  npm ERR! Tell the author that this fails on your system:
  npm ERR!     node node_modules/cross-env/bin/cross-env.js NODE_ENV=production node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js
  npm ERR! You can get their info via:
  npm ERR!     npm owner ls
  npm ERR! There is likely additional logging output above.
  
  npm ERR! Please include the following file with any support request:
  npm ERR!     /home/vagrant/projects/nielsvroman/npm-debug.log
  </code></pre>
  
  <p>What could be the problem with this?</p>