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.
"not a string. For example, style={{marginRight: spacing + 'em'}} " +
          'when using JSX.',
      );
    });
    it('should track input values', () => {
      var container = document.createElement('div');
      var inst = ReactDOM.render(
        <input type="text" defaultValue="foo" />,
        container,
      );
      var tracker = inputValueTracking._getTrackerFromNode(inst);
      expect(tracker.getValue()).toEqual('foo');
    });
    it('should track textarea values', () => {
      var container = document.createElement('div');
      var inst = ReactDOM.render(<textarea defaultValue="foo" />, container);
      var tracker = inputValueTracking._getTrackerFromNode(inst);
      expect(tracker.getValue()).toEqual('foo');
    });
    it('should warn for children on void elements', () => {
      class X extends React.Component {
        render() {
          return <input>moo</input>;
        }
      }
      var container = document.createElement('div');
      expect(function() {
        ReactDOM.render(<X />, container);
      }).toThrowError(
        'input is a void element tag and must neither have `children` ' +