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.
ops.push(`Parent:${this.props.step} componentDidMount`);
        }
        componentDidUpdate() {
          ops.push(`Parent:${this.props.step} componentDidUpdate`);
        }
        componentWillUnmount() {
          ops.push(`Parent:${this.props.step} componentWillUnmount`);
        }
        render() {
          const {step} = this.props;
          return [
            <Child key="a" name={`normal[0]:${step}`} />,
            ReactDOM.unstable_createPortal(
              <Child key="b" name={`portal1[0]:${step}`} />,
              portalContainer1,
            ),
            <Child key="c" name={`normal[1]:${step}`} />,
            ReactDOM.unstable_createPortal(
              [
                <Child key="d" name={`portal2[0]:${step}`} />,
                <Child key="e" name={`portal2[1]:${step}`} />,
              ],
              portalContainer2,
            ),
          ];
        }
      }
      ReactDOM.render(<Parent step="a" />, container);
      expect(portalContainer1.innerHTML).toBe('<div>portal1[0]:a</div>');
      expect(portalContainer2.innerHTML).toBe(