Stack overflow in while loop inside `async`

This code causes a stack overflow:

```
async {
  while (true) {
    if (false) {
      await(Future.successful(()))
    }
  }
}
```

The generated `resume` method takes up a stack frame for each iteration of the loop.

Tested with 0.9.2 for scala 2.10. Has this been fixed in the master branch? I don't have time to check at the moment.
