Incorrect imported function `this` context using ES6 module

Code

``` ts
// export.ts
export function foo() {
  'use strict'
  console.log(this);
}

foo();
```

``` ts
// import.ts
import { foo } from './export';

foo();
```

expected

```
$ node import.js
undefined
undefined
```

actual

```
$ node import.js
undefined
{ foo: [Function: foo] }
```
