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