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.
return (str + '').replace(/([.?*+\^$\[\]\\(){}|-])/g, '\\$1');
    }
    function genMarkup(props) {
      return ReactDOMServer.renderToString(<div {...props} />);
    }
    function toHaveInnerhtml(actual, expected) {
      var re = '^' + quoteRegexp(expected) + '$';
      return new RegExp(re).test(actual);
    }
    it('should handle dangerouslySetInnerHTML', () => {
      var innerHTML = {__html: 'testContent'};
      expect(
        toHaveInnerhtml(
          genMarkup({dangerouslySetInnerHTML: innerHTML}),
          'testContent',
        ),
      );
    });
  });
  describe('mountComponent', () => {
    var mountComponent;
    beforeEach(() => {
      mountComponent = function(props) {
        var container = document.createElement('div');
        ReactDOM.render(<div {...props} />, container);
      };