Intersection types containing `string` cannot be used as an index

[wycats]

interface Marker {}

type Foo = string & Marker;

let x = {};
let y: Foo = "hello";

x[y] = 1;

The error is demonstrated in the TypeScript playground (http://goo.gl/STqLUU) and is still present as of the latest typescript@next.

[weswigham]

Duplicates #5599