Non-null assertion operator should work on index signature

[tinganho]

I think the following code should work:

interface A {
    [index: string]: string;
}
interface B {
    a: A;
}
let b: B = { } as any;
b.a.some!; // error? some

Because you have asserted it to be non-null and an index signature tells that A can contain more props than what is defined in A. It makes sense to let people assert non-null in those cases.

[gcnew]

I think #12596 is what you are after.