avm2: Replaced stubs in Proxy with new format

core/src/avm2/globals/flash/utils/proxy.rs

@@ -4,12 +4,14 @@ use crate::avm2::value::Value;
 use crate::avm2::Error;
 
 pub use crate::avm2::object::proxy_allocator;
+use crate::avm2_stub_method;
 
 pub fn is_attribute<'gc>(
-    _activation: &mut Activation<'_, 'gc>,
+    activation: &mut Activation<'_, 'gc>,
     _this: Option<Object<'gc>>,
     _args: &[Value<'gc>],
 ) -> Result<Value<'gc>, Error<'gc>> {
     // yes, this is supposed to be implemented
-    Err("Proxy.isAttribute is not implemented".into())
+    avm2_stub_method!(activation, "flash.utils.Proxy", "isAttribute");
+    Ok(Value::Undefined)
 }