How do you get `rollup` to include a JS lib?

Ionic Framework ionic-v3
1

I want to extend a class defined as a global in an external JS lib

npm install --save js-marker-clusterer

I think I have figured out the typescript gymnastics to transpile correctly, but I still cannot figure out how to configure rollup.config.js to include the js file. The JS lib is NOT a commonjs module so the rollup-plugin-commonjs doesn’t seem to work.

I’m using "@ionic/app-scripts": "^0.0.36",

see http://stackoverflow.com/questions/40225851/how-do-you-include-extend-a-js-lib-with-rollup-typescript-angular2