On line 33 in package.json. The 'main' key's value is 'dist/markercluster.js'
In the dist folder there is only markercluster.min.js.
This causes my require statement to fail to find the module.
So out of the box it doesn't work.
The work around was to change the package.json to 'dist/markercluster.min.js.'
I didn't want to bother with building the un-minified file. But I might want to though.
Anyways. Thanks for the great library!