switch case unreachable code checking when use string literals

Hello! See example

``` javascript
type lang = "en" | "ru";
function getDict(lng: lang): any {
    switch (lng) {
        case 'ru':
            return ru;
        case 'stringNotInLangType':
            return en;
    }
}
```

compiler should throw error, cause 

``` javascript
 case 'stringNotInLangType':
            return en;
```

is unreachable
