Goto definition should go to index signature

**TypeScript Version:**  master

**Code**

```ts
interface I {
    [x: string]: boolean;
}
declare const i: I;
i.foo;
```

**Expected behavior:**

Goto definition on `.foo` takes me to the index signature.

**Actual behavior:**

Takes me nowhere. This can be tough if you have a complicated type and have no idea where the index signature came from.