add back kind for backwards compatibility, until plugins can be staged onto $class

core/src/main/java/hudson/model/Descriptor.java

@@ -909,7 +909,11 @@ public abstract class Descriptor<T extends Describable<T>> implements Saveable {
         if (formData!=null) {
             for (Object o : JSONArray.fromObject(formData)) {
                 JSONObject jo = (JSONObject)o;
-                String kind = jo.getString("$class");
+                String kind = jo.optString("$class", null);
+                if (kind == null) {
+                  // Legacy: Remove once plugins have been staged onto $class
+                  kind = jo.getString("kind");
+                }
                 Descriptor<T> d = find(descriptors, kind);
                 if (d != null) {
                     items.add(d.newInstance(req, jo));

core/src/main/resources/lib/form/class-entry.jelly

@@ -38,6 +38,10 @@ THE SOFTWARE.
   <f:invisibleEntry>
     <!-- Legacy: Remove once plugins have been staged onto $class -->
     <input type="hidden" name="stapler-class" value="${clazz}" />
+    <!-- Legacy: Remove once plugins have been staged onto $class -->
+    <j:if test="${attrs.descriptor != null}">
+      <input type="hidden" name="kind" value="${attrs.descriptor.id}" />
+    </j:if>
     <input type="hidden" name="$class" value="${clazz}" />
   </f:invisibleEntry>
 </j:jelly>
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