title: undo 'npm run eject' in react
body: <p>I was trying to test the performance of my <em>React app</em>(created with create-react-app) with react CDN script and i did <strong>'npm run eject'</strong> to add <a href="https://webpack.js.org/configuration/externals/" rel="noreferrer">webpack externals</a> dependencies  react and react-dom. 
  <br/> <br/>
  I did that with ease in <strong>webpack config</strong> and <code>&lt;script&gt;</code> in <code>index.html</code></p>
  
  <pre><code>...
  externals: {
      react: 'React',
      'react-dom':'ReactDOM'
  },
  ...
  </code></pre>
  
  <p><strong>Now I want to revert it back to previous state</strong>
  <br><br>
  I am using <strong>git</strong> and i did this experiment in a seperate branch.</p>
  
  <p>I ran <code>git checkout master</code> and <code>npm start</code>
  <br><br>
  The result was annoying  </p>
  
  <pre><code>&gt; myapp@0.18.1 start /home/code/serverSync/myapp/ui
  &gt; react-scripts start
  
  sh: 1: react-scripts: not found
  
  npm ERR! Linux 4.15.0-23-generic
  npm ERR! argv "/usr/bin/node" "/usr/bin/npm" "start"
  npm ERR! node v8.10.0
  npm ERR! npm  v3.5.2
  npm ERR! file sh
  npm ERR! code ELIFECYCLE
  npm ERR! errno ENOENT
  npm ERR! syscall spawn
  npm ERR! myapp@0.18.1 start: `react-scripts start`
  npm ERR! spawn ENOENT
  npm ERR! 
  npm ERR! Failed at the myapp@0.18.1 start script 'react-scripts start'.
  npm ERR! Make sure you have the latest version of node.js and npm installed.
  npm ERR! If you do, this is most likely a problem with the myapp package,
  npm ERR! not with npm itself.
  npm ERR! Tell the author that this fails on your system:
  npm ERR!     react-scripts start
  npm ERR! You can get information on how to open an issue for this project with:
  npm ERR!     npm bugs myapp
  npm ERR! Or if that isn't available, you can get their info via:
  npm ERR!     npm owner ls myapp
  npm ERR! There is likely additional logging output above.
  
  npm ERR! Please include the following file with any support request:
  npm ERR!     /home/code/serverSync/myapp/ui/npm-debug.log
  </code></pre>
  
  <p>how can i revert back to master branch ?</p>