Ionic: webpackJsonp is not defined - ionic-framework

Question

Ionic app seems to have stopped working and I am not able pinpoint the cause. I started again from blank app, but this error persists
Not sure if I am missing setup steps or issue with version mismatch? any pointers would help as I have wasted weeks on this issue
Accepted answers, like -
WebpackJsonp missing after Ionic 3 update or Ionic 2: ReferenceError: webpackJsonp is not defined have not worked
index.html has
<!-- The polyfills js is generated during the build process -->
<script src="build/polyfills.js"></script>
<!-- The vendor js is generated during the build process
It contains all of the dependencies in node_modules -->
<script src="build/vendor.js"></script>
<!-- The main bundle js is generated during the build process -->
<script src="build/main.js"></script>
ionic info -
cli packages: (/usr/local/lib/node_modules)
#ionic/cli-utils : 1.19.2
ionic (Ionic CLI) : 3.20.0
global packages:
cordova (Cordova CLI) : 8.0.0
local packages:
#ionic/app-scripts : 3.1.8
Cordova Platforms : android 6.4.0 ios 4.5.4
Ionic Framework : ionic-angular 3.9.2
System:
Android SDK Tools : 26.1.1
ios-deploy : 1.9.2
Node : v9.5.0
npm : 5.7.1
service.worker.js has-
// pre-cache our key assets
self.toolbox.precache(
[
'./build/main.js',
'./build/vendor.js',
'./build/main.css',
'./build/polyfills.js',
'index.html',
'manifest.json'
]
);