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(
        <svg><text arabic-form="initial" /></svg>,
      );
      var text = el.querySelector('text');
      expect(text.hasAttribute('arabic-form')).toBe(true);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid DOM property `arabic-form`. Did you mean `arabicForm`?',
      );
    });
    it('sets aliased attributes on custom elements', function() {
      var el = ReactTestUtils.renderIntoDocument(
        <div is="custom-element" class="test" />,
      );
      expect(el.getAttribute('class')).toBe('test');
    });
    it('aliased attributes on custom elements with bad casing', function() {
      var el = ReactTestUtils.renderIntoDocument(
        <div is="custom-element" claSS="test" />,
      );
      expect(el.getAttribute('class')).toBe('test');
    });
    it('updates aliased attributes on custom elements', function() {
      var container = document.createElement('div');
      ReactDOM.render(<div is="custom-element" class="foo" />, container);
      ReactDOM.render(<div is="custom-element" class="bar" />, container);
      expect(container.firstChild.getAttribute('class')).toBe('bar');
    });
  });
  describe('Custom attributes', function() {
    it('allows assignment of custom attributes with string values', function() {