[unity] Disable baking with new prefab system.

spine-unity/Assets/Spine/Editor/spine-unity/Editor/SkeletonBakingWindow.cs

@@ -3,6 +3,11 @@ using System.Collections.Generic;
 using UnityEngine;
 using UnityEditor;
 
+#if UNITY_2018_3 || UNITY_2019
+#define NEW_PREFAB_SYSTEM
+#endif
+
+
 namespace Spine.Unity.Editor {
 
 	using Editor = UnityEditor.Editor;
@@ -11,6 +16,7 @@ namespace Spine.Unity.Editor {
 	public class SkeletonBakingWindow : EditorWindow {
 		const bool IsUtilityWindow = true;
 
+		#if !NEW_PREFAB_SYSTEM
 		[MenuItem("CONTEXT/SkeletonDataAsset/Skeleton Baking", false, 5000)]
 		public static void Init (MenuCommand command) {
 			var window = EditorWindow.GetWindow<SkeletonBakingWindow>(IsUtilityWindow);
@@ -20,6 +26,7 @@ namespace Spine.Unity.Editor {
 			window.skeletonDataAsset = command.context as SkeletonDataAsset;
 			window.Show();
 		}
+		#endif
 
 		public SkeletonDataAsset skeletonDataAsset;
 		[SpineSkin(dataField:"skeletonDataAsset")]