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.
state = {flip: false, count: 0};
        flip() {
          this.setState({flip: true, count: this.state.count + 1});
        }
        tick() {
          this.setState({count: this.state.count + 1});
        }
        render() {
          const useB = !this.props.forceA && this.state.flip;
          return <div onClick={useB ? handlerB : handlerA} />;
        }
      }
      class Click extends React.Component {
        constructor() {
          super();
          click(node);
        }
        render() {
          return null;
        }
      }
      let inst;
      ReactDOM.render([<Example key="a" ref={n => (inst = n)} />], container);
      const node = container.firstChild;
      expect(node.tagName).toEqual('DIV');
      function click(target) {