title: Not getting service worker with create-react-app
body: <p>lately, I have create two react application and in both of them the service worker is not present instead a new file is generated i.e 'reportWebVitals.js'.
  Here is my folder structure of src after creating new app.
  <a href="https://i.stack.imgur.com/3ZiWE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/3ZiWE.png" alt="File Structure" /></a></p>
  <p>Also, if we check on running the app, the browser says that 'No matching service worker detected'</p>