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.
// 
    const iteratorFn = getIteratorFn(newChildrenIterable);
    invariant(
      typeof iteratorFn === 'function',
      'An object is not an iterable. This error is likely caused by a bug in ' +
        'React. Please file an issue.',
    );
    if (__DEV__) {
      // 
      if (typeof newChildrenIterable.entries === 'function') {
        const possibleMap = (newChildrenIterable: any);
        if (possibleMap.entries === iteratorFn) {
          warning(
            didWarnAboutMaps,
            'Using Maps as children is unsupported and will likely yield ' +
              'unexpected results. Convert it to a sequence/iterable of keyed ' +
              'ReactElements instead.%s',
            getCurrentFiberStackAddendum(),
          );
          didWarnAboutMaps = true;
        }
      }
      // 
      // 
      const newChildren = iteratorFn.call(newChildrenIterable);
      if (newChildren) {
        let knownKeys = null;
        let step = newChildren.next();
        for (; !step.done; step = newChildren.next()) {
          const child = step.value;