Dict literal in TypedDict context

A compatible dict literal should be valid if the type context is a typed dict type. Example:

```python
from mypy_extensions import TypedDict

Point = TypedDict('Point', {'x': int, 'y': int})

def f(p: Point) -> None: pass

f({'x': 1, 'y': 3})  # Ok
p = {'x': 2, 'y': 3}  # type: Point  # Ok
```