Heroku only allows each dyno to send and receive external network traffic on a single port, which means you can't simply
run node --debug server.js and attach your debugger to your-app.herokuapp.com:5858.
To work around this, you can use ngrok and Heroku ngrok Buildpack to tunnel to the debugger's port and access it externally.
In this document, I use strings in the format "foo:bar" to represent ResourceLocations with domain foo and path bar. I also use [square brackets] for placeholders.
On startup and whenever the resources are reloaded (in ModelLoader#setupModelRegistry), Minecraft iterates through every registered Block (in ModelLoader#loadBlocks) and asks its custom IStateMapper (or DefaultStateMapper if none has been registered) to create a mapping between every valid IBlockState of the Block and the ModelResourceLocation for that state (with the domain and path pointing to a blockstates file and the variant to a variant within that file). It then attempts to load these models.
DefaultStateMapper looks for the blockstates file with the Block's registry name (i.e. assets/[modid]/blockstates/[name].json) and serialises each property and value of the IBlockState to create the variant name that the model is loaded from (e.g. "enabled=true,type=foobar"
This file is no longer used, I have transitioned from p3lim's AddOn Packager Proxy to Travis CI and the BigWigs Packager Script.