This was made by decompiling UnityEditor.dll and UnityEngine.dll using ILSpy

Current version: 5.6.0f3 - other versions can be found in the tags

Feel free to either browse the code directly or download it

Note that Unity UI is open source. Get it from https://bitbucket.org/Unity-Technologies/ui/

Permission here: https://twitter.com/aras_p/status/568126504997138434