When you forget to extend `React.Component` you get the following error:

```
TypeError: Cannot call a class as a function
    at _classCallCheck (null.js:7:99)
    at Hello (null.js:11:5)
    ...react internals...
```

[[Example](http://react.jsbin.com/?html,js,console)]

It would be nice if in dev you could do a check for `Component.prototype.render` and if it exists, provide a better warning along the lines of:

```
Warning: Using a class with a render method as a function, did you forget to extend React.Component?
``` 

I believe it would happen in [ReactCompositeComponent.js](https://github.com/facebook/react/blob/b840229286ac2a82fa49553ce793cf7b953d1845/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js#L248).
});
  it('should react to state changes from callbacks', () => {
    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
    var el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('A');
    ReactTestUtils.Simulate.click(el);
    el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('B');
  });
  it('should rewire refs when rendering to different child types', () => {
    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
    expect(ReactDOM.findDOMNode(instance.refs.x).tagName).toBe('A');
    instance._toggleActivatedState();
    expect(ReactDOM.findDOMNode(instance.refs.x).tagName).toBe('B');
    instance._toggleActivatedState();
    expect(ReactDOM.findDOMNode(instance.refs.x).tagName).toBe('A');
  });
  it('should not cache old DOM nodes when switching constructors', () => {
    var container = document.createElement('div');
    var instance = ReactDOM.render(
      <ChildUpdates renderAnchor={true} anchorClassOn={false} />,
      container,
    );
    ReactDOM.render(
      
      <ChildUpdates renderAnchor={true} anchorClassOn={true} />,
      container,
    );
    ReactDOM.render(