Unity3d 加载UnityEngine.UI.dll 失败导致 Prefab 上 脚本丢失

Unity3d 加载UnityEngine.UI.dll 失败导致 Prefab 上 脚本丢失，引发各种 NullException

在 Unity 官方论坛找到的解决方法是重新加载 UnityEngine.UI.dll ，把下面的脚本扔到项目里，然后右键菜单中就会出现

using UnityEngine;
using System.Collections.Generic;
using UnityEditor;
using System.Text.RegularExpressions;
using System.IO;
using System.Text;

public class ReimportUnityEngineUI {
    [MenuItem("Assets/Reimport UI Assemblies", false, 100)]
    public static void ReimportUI() {
#if UNITY_4_6
        var path = EditorApplication.applicationContentsPath + "/UnityExtensions/Unity/GUISystem/{0}/{1}";
        var version = Regex.Match(Application.unityVersion, @"^[0-9]+\.[0-9]+\.[0-9]+").Value;
#elif UNITY_4_7
        var path = EditorApplication.applicationContentsPath + "/UnityExtensions/Unity/GUISystem/{0}/{1}";
        var version = Regex.Match(Application.unityVersion, @"^[0-9]+\.[0-9]+\.[0-9]+").Value;
#else
        var path = EditorApplication.applicationContentsPath + "/UnityExtensions/Unity/GUISystem/{1}";
        var version = string.Empty;
#endif
        string engineDll = string.Format(path, version, "UnityEngine.UI.dll");
        string editorDll = string.Format(path, version, "Editor/UnityEditor.UI.dll");
        ReimportDll(engineDll);
        ReimportDll(editorDll);

    }
    static void ReimportDll(string path) {
        if (File.Exists(path))
            AssetDatabase.ImportAsset(path, ImportAssetOptions.ForceUpdate | ImportAssetOptions.DontDownloadFromCacheServer);
        else
            Debug.LogError(string.Format("DLL not found {0}", path));
    }
}