composition during return?

If this is a syntactic sugar for generators consider the following:

```
function* g() {
  try {
    yield todo();
  }
  finally {
    yield cleanup();
  };
}
let o = g();
o.next();
o.return();
```

According to https://code.google.com/p/v8/issues/detail?id=3133 cleanup() should not yield. For some reason this sounds wrong to me, but looks true to spec.
