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.
});
describe('disableNewFiberFeatures', () => {
  var container;
  var ReactFeatureFlags;
  beforeEach(() => {
    container = document.createElement('div');
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = true;
  });
  afterEach(() => {
    ReactFeatureFlags = require('ReactFeatureFlags');
    ReactFeatureFlags.disableNewFiberFeatures = false;
  });
  it('throws if non-element passed to top-level render', () => {
    const message = 'render(): Invalid component element.';
    expect(() => ReactDOM.render(null, container)).toThrow(message, container);
    expect(() => ReactDOM.render(undefined, container)).toThrow(
      message,
      container,
    );
    expect(() => ReactDOM.render(false, container)).toThrow(message, container);
    expect(() => ReactDOM.render('Hi', container)).toThrow(message, container);
    expect(() => ReactDOM.render(999, container)).toThrow(message, container);
    expect(() => ReactDOM.render([<div key="a" />], container)).toThrow(
      message,
      container,
    );
  });
  it('throws if something other than false, null, or an element is returned from render', () => {
    function Render(props) {