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.
static childContextTypes = {
          foo: PropTypes.string.isRequired,
          getFoo: PropTypes.func.isRequired,
        };
        state = {
          bar: 'initial',
        };
        getChildContext() {
          return {
            foo: this.state.bar,
            getFoo: () => this.state.bar,
          };
        }
        render() {
          return ReactDOM.unstable_createPortal(<Component />, portalContainer);
        }
      }
      var instance = ReactDOM.render(<Parent />, container);
      expect(portalContainer.innerHTML).toBe('<div>initial-initial</div>');
      expect(container.innerHTML).toBe('');
      instance.setState({bar: 'changed'});
      expect(portalContainer.innerHTML).toBe('<div>changed-changed</div>');
      expect(container.innerHTML).toBe('');
    });
    it('should update portal context if it changes due to re-render', () => {
      var portalContainer = document.createElement('div');
      class Component extends React.Component {