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.

[mhegazy]

Should be doable. getFlowTypeOfReference operates on nodes, so we should be able to pass in the node, along with the declared type from the index signature.

[sandersn]

Fix is up at #19838.