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 <div><Child1 /><Child2 /><Child3 /><Child4 /></div>;
        }
      }
      class Child1 extends React.Component {
        render() {
          return <div class="paladin">Child1</div>;
        }
      }
      class Child2 extends React.Component {
        render() {
          return <div>Child2</div>;
        }
      }
      class Child3 extends React.Component {
        render() {
          return <div onclick="1">Child3</div>;
        }
      }
      class Child4 extends React.Component {