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 '[object HTMLUnknownElement]';
          }
          return realToString.apply(this, arguments);
        };
        Object.prototype.toString = wrappedToString; 
        ReactTestUtils.renderIntoDocument(<bar />);
        
        ReactTestUtils.renderIntoDocument(<foo />);
        ReactTestUtils.renderIntoDocument(<foo />);
        
        
        
        
        
        ReactTestUtils.renderIntoDocument(<time />);
        
        ReactTestUtils.renderIntoDocument(<hasOwnProperty />);
      } finally {
        Object.prototype.toString = realToString; 
      }
      expectDev(console.error.calls.count()).toBe(4);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'The tag <bar> is unrecognized in this browser',
      );
      expectDev(console.error.calls.argsFor(1)[0]).toContain(
        'The tag <foo> is unrecognized in this browser',
      );
      expectDev(console.error.calls.argsFor(2)[0]).toContain(
        '<hasOwnProperty /> is using uppercase HTML',
      );