Yeah, I found this too. It think it might be a bug, or a initialization method that was removed and the documentation wasn't properly updated.
You can work around the issue with the following code:
var quat = Quat()
quat.set_euler(Vector3(1, 1, 1))
Or something like that. If the code above doesn't work, let me know and I'll find the snippet in one of my projects. It is not ideal, but it works around the initialization issue. Hopefully this helps :smile: