Incorrect type narrowing for TypedDicts using the `S in D` type guard pattern

[erictraut]

The narrowed type in the following example is incorrect:

class A(TypedDict):
    a: int

def func(t: A):
    if "b" in t:
        reveal_type(t) # Should be `A`, not `Never`

We can't eliminate type A based on this check because the TypedDict is not closed.