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.
'Warning: Invalid prop `foo` on <div> tag. Either remove this prop ' +
          'from the element, or pass a string or number value to keep ' +
          'it in the DOM. For details, see https://fb.me/react-unknown-prop' +
          '\n    in div (at **)',
      );
    });
    it('should group multiple unknown prop warnings together', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div foo={() => {}} baz={() => {}} />, container);
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid props `foo`, `baz` on <div> tag. Either remove these ' +
          'props from the element, or pass a string or number value to keep ' +
          'them in the DOM. For details, see https://fb.me/react-unknown-prop' +
          '\n    in div (at **)',
      );
    });
    it('should warn for onDblClick prop', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div onDblClick={() => {}} />, container);
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown event handler property `onDblClick`. Did you mean `onDoubleClick`?\n    in div (at **)',
      );
    });
    it('should not warn for "0" as a unitless style value', () => {
      spyOn(console, 'error');
      class Component extends React.Component {