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).
return {
        render() {
          return <div>{test}</div>;
        },
      };
    }
    var el = document.createElement('div');
    ReactDOM.render(<Child test="test" />, el);
    expect(el.textContent).toBe('test');
  });
  it('should support rendering to different child types over time', () => {
    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
    var el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('A');
    instance._toggleActivatedState();
    el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('B');
    instance._toggleActivatedState();
    el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('A');
  });
  it('should not thrash a server rendered layout with client side one', () => {
    class Child extends React.Component {