https://jsfiddle.net/3apqnhmw/

```jsx
class Foo extends React.Component {
  render() {
     return <div>Foo</div>
  }
}

class App extends React.Component {
  render() {
    return Foo
  }
}
```

When `App` returns `Foo` instead of `<Foo />` accidentally, Fiber does not catch this.
'React Component in warnForMissingKey should have a _store. ' +
        'This error is likely caused by a bug in React. Please file an issue.',
    );
    child._store.validated = true;
    var currentComponentErrorInfo =
      'Each child in an array or iterator should have a unique ' +
      '"key" prop. See https://fb.me/react-warning-keys for ' +
      'more information.' +
      (getCurrentFiberStackAddendum() || '');
    if (ownerHasKeyUseWarning[currentComponentErrorInfo]) {
      return;
    }
    ownerHasKeyUseWarning[currentComponentErrorInfo] = true;
    warning(
      false,
      'Each child in an array or iterator should have a unique ' +
        '"key" prop. See https://fb.me/react-warning-keys for ' +
        'more information.%s',
      getCurrentFiberStackAddendum(),
    );
  };
}
const {
  createWorkInProgress,
  createFiberFromElement,
  createFiberFromFragment,
  createFiberFromText,
  createFiberFromCoroutine,
  createFiberFromYield,
  createFiberFromPortal,