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).
ReactDOM.render(<Component flipped={true} />, container);
    expect(ReactDOM.findDOMNode(comp.refs.static0).textContent).toBe('B');
    expect(ReactDOM.findDOMNode(comp.refs.static1).textContent).toBe('A');
  });
  it('should allow access to findDOMNode in componentWillUnmount', () => {
    var a = null;
    var b = null;
    class Component extends React.Component {
      componentDidMount() {
        a = ReactDOM.findDOMNode(this);
        expect(a).not.toBe(null);
      }
      componentWillUnmount() {
        b = ReactDOM.findDOMNode(this);
        expect(b).not.toBe(null);
      }
      render() {
        return <div />;
      }
    }
    var container = document.createElement('div');
    expect(a).toBe(container.firstChild);
    ReactDOM.render(<Component />, container);
    ReactDOM.unmountComponentAtNode(container);
    expect(a).toBe(b);
  });
  it('context should be passed down from the parent', () => {
    class Parent extends React.Component {