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.
var el = ReactTestUtils.renderIntoDocument(<div whatever="30" />);
      expect(el.getAttribute('whatever')).toBe('30');
    });
    it('removes custom attributes', function() {
      const container = document.createElement('div');
      ReactDOM.render(<div whatever="30" />, container);
      expect(container.firstChild.getAttribute('whatever')).toBe('30');
      ReactDOM.render(<div whatever={null} />, container);
      expect(container.firstChild.hasAttribute('whatever')).toBe(false);
    });
    it('does not assign a boolean custom attributes as a string', function() {
      spyOn(console, 'error');
      var el = ReactTestUtils.renderIntoDocument(<div whatever={true} />);
      expect(el.hasAttribute('whatever')).toBe(false);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid prop `whatever` on <div> tag',
      );
    });
    it('does not assign an implicit boolean custom attributes', function() {
      spyOn(console, 'error');
      
      var el = ReactTestUtils.renderIntoDocument(<div whatever />);
      expect(el.hasAttribute('whatever')).toBe(false);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid prop `whatever` on <div> tag',
      );
    });
    it('assigns a numeric custom attributes as a string', function() {
      var el = ReactTestUtils.renderIntoDocument(<div whatever={3} />);
      expect(el.getAttribute('whatever')).toBe('3');