No type inference for dicts in union

```py
from typing import *

def foo() -> Dict[str, Any]:
    return {}

def bar() -> Union[Dict[str, Any], Dict[int, Any]]:
    return {}
```

In `foo`, `{}` is inferred as `Dict[str, Any]`.
In `bar`, I get:
```
error: Incompatible return value type (got "Dict[<nothing>, <nothing>]", expected "Union[Dict[str, Any], Dict[int, Any]]")
```