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.
expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<BR /> is using uppercase HTML.',
      );
    });
    it('should warn on upper case HTML tags, not SVG nor custom tags', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        React.createElement('svg', null, React.createElement('PATH')),
      );
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('CUSTOM-TAG'));
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('IMG'));
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<IMG /> is using uppercase HTML.',
      );
    });
    it('should warn if the tag is unrecognized', () => {
      spyOn(console, 'error');
      let realToString;
      try {
        realToString = Object.prototype.toString;
        let wrappedToString = function() {
          
          if (this instanceof window.HTMLUnknownElement) {
            return '[object HTMLUnknownElement]';
          }
          
          if (this instanceof window.HTMLTimeElement) {