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.
render() {
          return <span />;
        }
        componentWillUnmount() {
          
          expect(ReactDOM.findDOMNode(this).nodeName).toBe('SPAN');
        }
      }
      var container = document.createElement('div');
      ReactDOM.render(<div><Inner /></div>, container);
      ReactDOM.unmountComponentAtNode(container);
    });
  });
  describe('tag sanitization', () => {
    it('should throw when an invalid tag name is used server-side', () => {
      var hackzor = React.createElement('script tag');
      expect(() => ReactDOMServer.renderToString(hackzor)).toThrowError(
        'Invalid tag: script tag',
      );
    });
    it('should throw when an attack vector is used server-side', () => {
      var hackzor = React.createElement('div><img /><div');
      expect(() => ReactDOMServer.renderToString(hackzor)).toThrowError(
        'Invalid tag: div><img /><div',
      );
    });
    it('should throw when an invalid tag name is used', () => {
      var hackzor = React.createElement('script tag');
      expect(() => ReactTestUtils.renderIntoDocument(hackzor)).toThrow();
    });