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 'hello';
        },
      };
      var container = document.createElement('div');
      ReactDOM.render(<img src={obj} />, container);
      expect(container.firstChild.src).toBe('hello');
      ReactDOM.render(<svg arabicForm={obj} />, container);
      expect(container.firstChild.getAttribute('arabic-form')).toBe('hello');
      ReactDOM.render(<div customAttribute={obj} />, container);
      expect(container.firstChild.getAttribute('customAttribute')).toBe(
        'hello',
      );
    });