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.
expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(<div onfocusin={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactTestUtils.renderIntoDocument(<div onfocusout={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('should warn about props that are no longer supported (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(<div />);
      expectDev(console.error.calls.count()).toBe(0);
      ReactDOMServer.renderToString(<div onFocusIn={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactDOMServer.renderToString(<div onFocusOut={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('should warn about props that are no longer supported without case sensitivity (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(<div />);
      expectDev(console.error.calls.count()).toBe(0);
      ReactDOMServer.renderToString(<div onfocusin={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactDOMServer.renderToString(<div onfocusout={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('gives source code refs for unknown prop warning', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div class="paladin" />);
      ReactTestUtils.renderIntoDocument(<input type="text" onclick="1" />);
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(