Virtual subclasses of abstract base classes

Seeing a couple of different errors with virtual subclasses:

``` python
from abc import ABCMeta
class M(metaclass=ABCMeta):
    pass
class A:
    pass
M.register(A)  # error: "M" has no attribute "register"
def test(obj: M):
    pass
obj = A()
test(obj)  # error: Argument 1 to "test" has incompatible type "A"; expected "M"

```
