Hello @kartik,
If your package.json looks something like the following:
  ...
  "devDependencies": {
    "babel": "^6.5.2",
    "babel-eslint": "^6.0.4",
    "babel-polyfill": "^6.8.0",
    "babel-preset-es2015": "^6.6.0",
    "babelify": "^7.3.0",
  ...
And you get the Cannot find module 'babel/polyfill' error message, then you probably just need to change your import statement FROM:
import "babel/polyfill";
TO:
import "babel-polyfill";
And make sure it comes before any other import statement 
Hope it is helpful!!
Thank you!!