[Truffle] Fix handling of CoreMethod.constructor.

* The method should be defined on the singleton class.

Author: eregon

truffle/src/main/java/org/jruby/truffle/nodes/core/CoreMethodNodeManager.java

@@ -102,17 +102,23 @@ private void addCoreMethod(MethodDetails methodDetails) {
             if (method.onSingleton()) {
                 System.err.println("WARNING: Either onSingleton or isModuleFunction for " + methodDetails.getIndicativeName());
             }
+            if (method.constructor()) {
+                System.err.println("WARNING: Either constructor or isModuleFunction for " + methodDetails.getIndicativeName());
+            }
             if (!module.isOnlyAModule()) {
                 System.err.println("WARNING: Using isModuleFunction on a Class for " + methodDetails.getIndicativeName());
             }
         }
+        if (method.onSingleton() && method.constructor()) {
+            System.err.println("WARNING: Either onSingleton or isModuleFunction for " + methodDetails.getIndicativeName());
+        }
 
         final RubyRootNode rootNode = makeGenericMethod(context, methodDetails);
 
         if (method.isModuleFunction()) {
             addMethod(module, rootNode, names, Visibility.PRIVATE);
             addMethod(getSingletonClass(module), rootNode, names, Visibility.PUBLIC);
-        } else if (method.onSingleton()) {
+        } else if (method.onSingleton() || method.constructor()) {
             addMethod(getSingletonClass(module), rootNode, names, visibility);
         } else {
             addMethod(module, rootNode, names, visibility);