Changes to node_modules should trigger recompilation

Created by: gaearon

1. Add import 'react-router' to index.js
2. See module not found error
3. npm install react-router
4. Still see module not found error
5. Press “save” in index.js
6. Now it compiled successfully!

Steps 4 and 5 should not be there. We should find a way to tell webpack that node_modules have changed, and it’s worth trying to recompile.

Ideally we should not have situations where user is forced to stop the bundler and run npm start again. I remember having such issues when switching branches. Hopefully fixing the above use case should be enough, but if you’re familiar with other cases where a restart is required, please write in this thread.