Use return type as an inference location

From https://github.com/Microsoft/TypeScript/issues/11054

``` ts
interface FolderContentItem{
    type: 'folder' | 'file';
}

let a:FolderContentItem[] = [];
a = [1,2,3,4,5].map(v=>({type:'folder'}))
```

Today this is an error without casting `"folder"` to the literal type. We have a contextual type coming from `a` that is not being used.
