Assets/Editor/UpdateTreeColors.js(13,17): BCE0031: Language feature not implemented: UnityEditor.

It's for the Island Demo on unity, I'd like to get it working but I keep getting these errors and I know some Javascript but all in all I'm rather inexperienced.

Here's my code.

 @MenuItem("Terrain/Update Tree Lightmap Color")
 static function RebuildWithLightmap () {
     var tex : Texture2D = Selection.activeObject as Texture2D;
     if (tex)
     {
         if (Terrain.activeTerrain == null || Terrain.activeTerrain.terrainData == null)
         {
             EditorUtility.DisplayDialog("No active terrain in the scene", "No active terrain in the scene", "Ok");
             return;
         }
         
         Undo.RegisterUndo(Terrain.activeTerrain.terrainData, "Set Tree colors");
         UnityEditor.TerrainLightmapper.UpdateTreeLightmapColor(tex, Terrain.activeTerrain.terrainData);
     }
     else
         EditorUtility.DisplayDialog("Select a lightmap", "Select a lightmap", "Ok");
 }
 
 @MenuItem("Terrain/Update Tree Color")
 static function RebuildWithColor () {
     var tex : Texture2D = Selection.activeObject as Texture2D;
     if (tex)
     {
         if (Terrain.activeTerrain == null || Terrain.activeTerrain.terrainData == null)
         {
             EditorUtility.DisplayDialog("No active terrain in the scene", "No active terrain in the scene", "Ok");
             return;
         }
         
         Undo.RegisterUndo(Terrain.activeTerrain.terrainData, "Set Tree colors");
         UnityEditor.TerrainLightmapper.UpdateTreeColor(tex, Terrain.activeTerrain.terrainData);
     }
     else
         EditorUtility.DisplayDialog("Select a lightmap", "Select a lightmap", "Ok");
 }

So if anyone can help that'd be great. It says it's on this line:

 UnityEditor.TerrainLightmapper.UpdateTreeLightmapColor(tex, Terrain.activeTerrain.terrainData);

Thanks!