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.
componentDidMount() {
        this.props.children[1] = <p key={1} />; 
        this.forceUpdate();
      }
      render() {
        return <div>{this.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 support refs on owned components', () => {
    var innerObj = {};
    var outerObj = {};
    class Wrapper extends React.Component {