Angular4.x Module not found: Error: Can't resolve 'classlist.js'

remove the current package,

npm uninstall classlist.js --save

then run

npm cache verify

afterwards run

npm install classlist.js --save-exact

You're importing a polyfill that is not included by default. You have to install it as a dependency first. How to do that is in a comment on the same line that imports it:

// Run `npm install --save classlist.js`.

The same goes for the web-animations-js polyfill if you haven't done so already.