Doesn't seem to detect reference updates (equal, but not identical)

``` js
reducer(state = {}, action) {
  // I would expect this to trigger an error, but it doesn't
  state.foo = {};
  return state;
}
```

One I give it differing values, it does:

``` js
let count = 0;
reducer(state = {}, action) {
  // boom
  state.foo = {count: count++};
  return state;
}
```
