Free functions always have a `prototype`, this is a holdover from ES3.

core/src/avm2.rs

@@ -1313,13 +1313,21 @@ impl<'gc> Avm2<'gc> {
         let method_entry = self.table_method(index)?;
         let scope = self.current_stack_frame().unwrap().read().scope();
 
-        let new_fn = FunctionObject::from_abc_method(
+        let mut new_fn = FunctionObject::from_abc_method(
             context.gc_context,
             method_entry,
             scope,
             self.system_prototypes.function,
             None,
         );
+        let es3_proto = ScriptObject::object(context.gc_context, self.prototypes().object);
+
+        new_fn.install_slot(
+            context.gc_context,
+            QName::new(Namespace::public_namespace(), "prototype"),
+            0,
+            es3_proto.into(),
+        );
 
         self.push(new_fn);
 

core/src/avm2/object.rs

@@ -508,13 +508,21 @@ pub trait TObject<'gc>: 'gc + Collect + Debug + Into<Object<'gc>> + Clone + Copy
                 slot_id, function, ..
             } => {
                 let method = Avm2MethodEntry::from_method_index(abc, function.clone()).unwrap();
-                let function = FunctionObject::from_abc_method(
+                let mut function = FunctionObject::from_abc_method(
                     context.gc_context,
                     method,
                     scope,
                     fn_proto,
                     None,
                 );
+                let es3_proto = ScriptObject::object(context.gc_context, avm.prototypes().object);
+
+                function.install_slot(
+                    context.gc_context,
+                    QName::new(Namespace::public_namespace(), "prototype"),
+                    0,
+                    es3_proto.into(),
+                );
                 self.install_const(context.gc_context, trait_name, *slot_id, function.into());
             }
             AbcTraitKind::Const { slot_id, value, .. } => {