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 <polymer-component />;
          }
        }
        var node = document.createElement('div');
        ReactDOM.render(<ShadyComponent />, node);
        expectDev(console.error.calls.count()).toBe(1);
        expectDev(console.error.calls.argsFor(0)[0]).toContain(
          'ShadyComponent is using shady DOM. Using shady DOM with React can ' +
            'cause things to break subtly.',
        );
        mountComponent({is: 'custom-shady-div2'});
        expectDev(console.error.calls.count()).toBe(1);
      } finally {
        document.createElement = defaultCreateElement;
      }
    });
    it('should emit a warning once for an unnamed custom component using shady DOM', () => {
      spyOn(console, 'error');
      var defaultCreateElement = document.createElement.bind(document);
      try {
        document.createElement = element => {
          var container = defaultCreateElement(element);
          container.shadyRoot = {};
          return container;
        };
        mountComponent({is: 'custom-shady-div'});