We used to throw if event listener is not a function in Stack:

```js
ReactDOM.render(
  <h1 onClick={{}}>Hello World!</h1>,
  document.getElementById('container')
);
```

<img width="565" alt="screen shot 2017-08-08 at 3 41 08 pm" src="https://user-images.githubusercontent.com/810438/29077502-2646412e-7c50-11e7-8c99-3b7f6c0a2589.png">

Fiber silently ignores this.

I think we should throw. This would let us add support for objects (e.g. for passive listener configuration) if we want to in a minor rather than a major.
return props.children;
    }
    expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(
      /You may have returned undefined/,
    );
    expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(
      /You may have returned undefined/,
    );
    expect(() =>
      ReactDOM.render(<Render>[<div key="a" />]</Render>, container),
    ).toThrow(/You may have returned undefined/);
  });
  it('treats mocked render functions as if they return null', () => {
    class Mocked extends React.Component {}
    Mocked.prototype.render = jest.fn();
    ReactDOM.render(<Mocked />, container);
    expect(container.textContent).toEqual('');
  });
  it('throws if the React package cannot be loaded', () => {
    jest.resetModules();
    jest.mock('react', () => undefined);
    expect(() => require('react-dom')).toThrow(
      'ReactDOM was loaded before React.',
    );
    jest.resetModules();
  });
});