"Extras" is a directory where the script "ClusterLayer.js" is found. If you download the source code from its sample page, you'll see how the directory structure for the project is set up. The location of this directory is set in the dojoConfig portion of the index.html. You can read up more about how to configure the paths or packages here: Configuring Dojo with dojoConfig - Dojo Toolkit Tutorial
I've to download the sample and use that .js inside of the .zip or there's a pack or something like that to download? You know if it has compatibility with v3.11?