--check-untyped-defs causes a crash if self is missing

This test case currently crashes with `AssertionError` on `assert len(arg_types) == len(arg_kinds) == len(arg_names)`:
```
[case testCheckUntypedDefsSelf3]
# flags: --check-untyped-defs

class Foo:
    def bad():
        self.x = 0
```
I think this may be a regression.