You can see the runtime.js of the built in plugins on c3 to see how they work through the chrome debugger. That is what I have done to see how a few things are done.
You just have to run your game in preview and open the chrome debugger on the preview window.
Go to the source tab
Expand the edit.construct.net part of the tree
Expand r85, or whichever version you happen to be on.
Expand the behaviors or plugins sections.
Each behavior or plugins runtime.js file will under separate folders. You can click on it, set break points, inspect variables, and get ideas on how things work.
*Note: If you start to create your own under developer mode and a local http server, those runtime ones will appear in the one of the entries under (no domain) and will be in one of the blob items.
Hope this helps
I forgot to mention that you will only see the plugins and behaviors that you have included in the project. Also it helps to create a test project with only one of the item you want to debug. This makes it easier to debug so you don't have multiple objects calling the same runtime script.