Disable service worker by default (facebook#3817)

* WIP disable service worker by default (facebook#2554)

* Updated service worker registration

* Readd default export in registerServiceWorker.js

* Updated comments about using Service Worker

* Call it serviceWorker

* Nits

packages/react-scripts/template/src/index.js

@@ -2,7 +2,11 @@ import React from 'react';
 import ReactDOM from 'react-dom';
 import './index.css';
 import App from './App';
-import registerServiceWorker from './registerServiceWorker';
+import * as serviceWorker from './serviceWorker';
 
 ReactDOM.render(<App />, document.getElementById('root'));
-registerServiceWorker();
+
+// If you want your app to work offline and load faster, you can change
+// unregister() to register() below. Note this comes with some pitfalls.
+// Learn more about service workers: http://bit.ly/2vJdu84
+serviceWorker.unregister();

packages/react-scripts/template/src/registerServiceWorker.js → packages/react-scripts/template/src/serviceWorker.js

@@ -18,7 +18,7 @@ const isLocalhost = Boolean(
     )
 );
 
-export default function register() {
+export function register() {
   if (process.env.NODE_ENV === 'production' && 'serviceWorker' in navigator) {
     // The URL constructor is available in all browsers that support SW.
     const publicUrl = new URL(process.env.PUBLIC_URL, window.location);