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 [<NullComponent key="a" />, <div key="b" />, <span key="c" />];
        }
      }
      let instance = null;
      ReactDOM.render(<Fragment ref={ref => (instance = ref)} />, container);
      expect(container.childNodes.length).toBe(2);
      const firstNode = ReactDOM.findDOMNode(instance);
      expect(firstNode).toBe(container.firstChild);
      expect(firstNode.tagName).toBe('DIV');
    });
  }
  if (ReactDOMFeatureFlags.useFiber) {
    var svgEls, htmlEls, mathEls;
    var expectSVG = {ref: el => svgEls.push(el)};
    var expectHTML = {ref: el => htmlEls.push(el)};
    var expectMath = {ref: el => mathEls.push(el)};
    var usePortal = function(tree) {
      return ReactDOM.unstable_createPortal(
        tree,
        document.createElement('div'),
      );
    };
    var assertNamespacesMatch = function(tree) {
      container = document.createElement('div');
      svgEls = [];
      htmlEls = [];
      mathEls = [];
      ReactDOM.render(tree, container);
      svgEls.forEach(el => {