UnityEngine.UI.dll is in timestamps but is not known in assetdatabase

项目启动后莫名的崩溃，各种UI组件都加载不出来了，报错： UnityEngine.UI.dll is in timestamps but is not known in assetdatabase。

心塞，， ， 终于找到一个方案，感谢  j.robichaud ，

1. Remove the UI System
   1. Close Unity
   2. Access on Unity's installation folder on Explorer or Finder on mac (On mac rightclick on the App and click on ShowPackageContent).
   3. Locate the folder named "UnityExtensions/Unity". (On Mac: "Content/UnityExtensions/Unity".
   4. In this folder you will notice a folder named "GUISystem"
   5. Move this folder from the installation folder (you may need administrator rights). Yes, right. Do not just rename it or move it out the "Unity" folder, Unity will find it... I assure you...
2. Have Unity notice the change
   1. Open Unity again in the project you had the issue
   2. You may have an error "GameObject (named 'Canvas') references runtime script in scene file. Fixing!"
   3. Close Unity, DO NO SAVE ANY SCENE
3. Restore the UI System
   1. Make sure Unity is closed
   2. Restore the "GUISystem" to its previous location
   3. Open Unity again
   4. Unity will compile script
   5. You will see the warning "Timestamps (19) and assets (21) maps out of sync." and the errors "Asset '' is in assets but has no assettimestamp..." just ignore them
      
   6. Everything should be ok again

参考地址： http://forum.unity3d.com/threads/unityengine-ui-dll-is-in-timestamps-but-is-not-known-in-assetdatabase.274492/