Unity2017.4.30f1 without GUISystem
1.删除了Unity安装目录下的 Editor/版本号/Editor/Data/UnityExtensions/Unity/GUISystem 文件夹;
2.在项目Assets中导入源码的 UnityEngine.UI 和 UnityEditor.UI(导入后需要将文件夹名称改为 Editor)文件夹;
UGUI源码下载地址:https://bitbucket.org/Unity-Technologies/ui/downloads/?tab=tags