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.
props.children[1] = <p key={1} />; 
      return <div>{props.children}</div>;
    }
    expect(() => {
      ReactTestUtils.renderIntoDocument(
        <Wrapper>
          <span key={0} />
          <span key={1} />
          <span key={2} />
        </Wrapper>,
      );
    }).toThrowError(/Cannot assign to read only property.*/);
  });
  it('should warn when children are mutated during update', () => {
    spyOn(console, 'error');
    class Wrapper extends React.Component {