Non-null assertion type operator ! #14366

TypeScript Version: 2.2.1

Code

interface Foo {
    bar: {
        baz: number;
    } | null;
}

type Baz = Foo["bar"]["baz"];  // This doesn't compile with the error: "Property 'baz' does not exist on type '{ baz: number; } | null'."
let x: Baz = 0;

Expected behavior:
I would expect there to be a way to specify bar is non-null, such as Foo["bar!"].

Actual behavior:
The code doesn't compile with the following error message:
"Property 'baz' does not exist on type '{ baz: number; } | null'."