Mypy throws error using sorted with a key function specified

[timabbott]

Reproducer: sorted([{'name': 'test'}], key=lambda x: x['name'])

Moved from #1150.

[gvanrossum]

The error is: Value of type object is not indexable. It's about the `x['name'] in the lambda. Almost feels like the type variable leaked or got substituted wrong. Maybe an issue in infer_function_type_arguments()?

[timabbott]

Confirmed the fix in my application, thanks @JukkaL !