title: How to provide `babel-preset-react-app` env variables?
body: <p>I am working on an App which connects create-react-app with an express server( using server rendering). I am referring  <a href="https://medium.com/bucharestjs/upgrading-a-create-react-app-project-to-a-ssr-code-splitting-setup-9da57df2040a" rel="noreferrer">this tutorial</a> for it.To render the file from server, the code is</p>
  
  <p><strong>bootstrap.js</strong> </p>
  
  <pre><code>require('ignore-styles');
  require('babel-register')({
  ignore:[/(node-modules)/],
  presets:['es2015','react-app']
  });
  require('./index');
  </code></pre>
  
  <p><strong>index.js</strong></p>
  
  <pre><code>import express from 'express';
  // we'll talk about this in a minute:
  import serverRenderer from './middleware/renderer';
  
  
  const PORT = 3000;
  const path = require('path');
  // initialize the application and create the routes
  const app = express();
  const router = express.Router();
  // root (/) should always serve our server rendered page
  router.use('^/$', serverRenderer);
  // other static resources should just be served as they are
  router.use(express.static(
      path.resolve(__dirname, '..', 'build'),
      { maxAge: '30d' },
  ));
  // tell the app to use the above rules
  app.use(router);
  // start the app
  app.listen(PORT, (error) =&gt; {
      if (error) {
          return console.log('something bad happened', error);
      }
  
      console.log("listening on " + PORT + "...");
  });
  </code></pre>
  
  <p>While running the command </p>
  
  <pre><code>node bootstrap.js
  </code></pre>
  
  <p>I am getting error that</p>
  
  <blockquote>
    <p>Error: Using <code>babel-preset-react-app</code> requires that you specify <code>NODE_ENV</code> or <code>BABEL_ENV</code> environment variables. Valid values are "development", "test", and "production".</p>
  </blockquote>