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.
throw new Error('Hello');
      }
      expect(() => {
        assertNamespacesMatch(
          <svg {...expectSVG}>
            <BrokenRender />
          </svg>,
        );
      }).toThrow('Hello');
      assertNamespacesMatch(<div {...expectHTML} />);
    });
    it('should unwind namespaces on caught errors', () => {
      function BrokenRender() {
        throw new Error('Hello');
      }
      class ErrorBoundary extends React.Component {