We don't ship the .js files in the git master branch.
  They are only present in version tags and in npm.
  
  See how to get the JS files here: https://github.com/Leaflet/Leaflet.markercluster#using-the-plugin
  Or how to build them: https://github.com/Leaflet/Leaflet.markercluster#building-testing-and-linting-scripts