Forward reference to a type alias results in an "Invalid type ..." error

mypy 0.4.5 and 0.4.4, CPython 3.5.1 and 3.5.2.

Code:

``` python
# code.py
def f(p: 'Alias') -> None:
    print(p)

Alias = int
```

Expected result:

```
% mypy code.py
%
```

Actual result:

```
% mypy code.py
code.py: note: In function "f":
code.py:1: error: Invalid type "code.Alias"
%
```
