avm2: Don't add ClassDefinition to AMF0 objects

core/src/avm2/amf.rs

@@ -126,11 +126,15 @@ pub fn serialize_value<'gc>(
                     recursive_serialize(activation, o, &mut object_body, amf_version).unwrap();
                     Some(AmfValue::Object(
                         object_body,
+                        if amf_version == AMFVersion::AMF3 {
                             Some(ClassDefinition {
                                 name: "".to_string(),
                                 attributes: EnumSet::only(Attribute::Dynamic),
                                 static_properties: Vec::new(),
-                        }),
+                            })
+                        } else {
+                            None
+                        },
                     ))
                 } else {
                     tracing::warn!(

tests/tests/swfs/avm2/socket_read_write_object/test.toml

@@ -1,2 +1 @@
 num_ticks = 10
-known_failure = true # Flash serializes AMF0 Object as object, but ruffle as typed object