Regression in discriminated unions unioned with primitive

From `master`

``` ts
interface Foo {
  kind: "foo";
  name: string;
}
interface Bar {
  kind: "bar";
  length: string;
}

let x: string | Foo | Bar = <any>undefined;
if(typeof x === 'string') {

} else {
  switch(x.kind) {
    case 'foo':
      console.log(x.name); // <-- Error, Property 'name' does not exist on type 'Foo | Bar'.
  }
}
```

Compiles without error in 2.1.0-dev.20160729
