2.0: Type parameters not instantiated in construction of class extended with interface in another file

a.ts:

``` ts
declare class P<R> {
    constructor(callback: (resolve: (value: R) => void) => void);
}
```

b.ts:

``` ts
interface P<R> {}
new P<string>(resolve => {
    resolve('foo'); // "Argument of type 'string' is not assignable to parameter of type 'R'."
});
```

According to git bisect, this was introduced in https://github.com/Microsoft/TypeScript/pull/7448
