Allow narrowing properties retrieved from types with index signatures

[kujon]

TypeScript Version: 2.4.0

Code

type A = {
 [index: string]: [number, number] | null;
};

const value: A = {
 foo: [42, 42]
};

const extractedValue = value.foo !== null ?
 // Object is possibly 'null'.
 value.foo[0] :
 5;

Expected behavior:
Null check is correctly performed and the compiler allows for accessing

Actual behavior:
// Object is possibly 'null'.

[DanielRosenwasser]

This would require us to synthesize some sort of knowledge about a property that's never been declared on the type A.