Fix backward compatibility issue

Grails is using the method
diff --git a/src/main/java/org/codehaus/groovy/classgen/VariableScopeVisitor.java b/src/main/java/org/codehaus/groovy/classgen/VariableScopeVisitor.java
index 5fb5678..752267f 100644
--- a/src/main/java/org/codehaus/groovy/classgen/VariableScopeVisitor.java
+++ b/src/main/java/org/codehaus/groovy/classgen/VariableScopeVisitor.java
@@ -346,6 +346,13 @@
     }
 
     //--------------------------------------------------------------------------
+    /**
+     * Sets the current class node context.
+     */
+    public void prepareVisit(ClassNode node) {
+        currentClass = node;
+        currentScope.setClassScope(node);
+    }
 
     @Override
     public void visitClass(final ClassNode node) {