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.
</svg>
            <image {...expectSVG} />
          </svg>
          <p {...expectHTML} />
        </div>,
      );
      assertNamespacesMatch(
        <div {...expectHTML}>
          <svg {...expectSVG}>
            <svg {...expectSVG}>
              <image {...expectSVG} />
              {usePortal(
                <svg {...expectSVG}>
                  <image {...expectSVG} />
                  <svg {...expectSVG}>
                    <image {...expectSVG} />
                  </svg>
                  <image {...expectSVG} />
                </svg>,
              )}
              <image {...expectSVG} />
              <foreignObject {...expectSVG}>
                <p {...expectHTML} />
                {usePortal(<p {...expectHTML} />)}
                <p {...expectHTML} />
              </foreignObject>
            </svg>
            <image {...expectSVG} />
          </svg>
          <p {...expectHTML} />