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.
it('removes a property when it becomes invalid', function() {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div whatever={0} />, container);
      ReactDOM.render(<div whatever={() => {}} />, container);
      var el = container.firstChild;
      expect(el.hasAttribute('whatever')).toBe(false);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid prop `whatever` on <div> tag.',
      );
    });
    it('warns on bad casing of known HTML attributes', function() {
      spyOn(console, 'error');
      var el = ReactTestUtils.renderIntoDocument(<div SiZe="30" />);
      expect(el.getAttribute('size')).toBe('30');
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid DOM property `SiZe`. Did you mean `size`?',
      );
    });
  });
  describe('Object stringification', function() {
    it('allows objects on known properties', function() {
      var el = ReactTestUtils.renderIntoDocument(
        <div allowTransparency={{}} />,
      );
      expect(el.getAttribute('allowtransparency')).toBe('[object Object]');
    });
    it('should pass objects as attributes if they define toString', () => {
      var obj = {
        toString() {