Change get_type_key to return an element rather than a set. It seems that the type doesn't match between the super class and the child class, and as I see no test failing this seems to have been a mistake.

[copybara-service[bot]]

Change get_type_key to return an element rather than a set. It seems that the type doesn't match between the super class and the child class, and as I see no test failing this seems to have been a mistake.

I presume this is just dead code, but this blocks annotating types on this function, and also bit hesitant to totally remove the code itself.