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.
container = document.createElement('div');
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = false;
  });
  afterEach(() => {
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = true;
  });
  it('should render strings as children', () => {
    const Box = ({value}) => <div>{value}</div>;
    ReactDOM.render(<Box value="foo" />, container);
    expect(container.textContent).toEqual('foo');
  });
  it('should render numbers as children', () => {
    const Box = ({value}) => <div>{value}</div>;
    ReactDOM.render(<Box value={10} />, container);
    expect(container.textContent).toEqual('10');
  });
  it('should be called a callback argument', () => {
    
    let called = false;
    ReactDOM.render(<div>Foo</div>, container, () => (called = true));
    expect(called).toEqual(true);
    
    called = false;
    ReactDOM.render(<div>Foo</div>, container, () => (called = true));
    expect(called).toEqual(true);
  });
  it('should call a callback argument when the same element is re-rendered', () => {
    class Foo extends React.Component {