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).
componentDidMount() {
        ReactDOM.render(<div />, layer);
      }
      componentWillUnmount() {
        ReactDOM.unmountComponentAtNode(layer);
      }
      render() {
        return <div />;
      }
    }
    class Outer extends React.Component {
      render() {
        return <div>{this.props.children}</div>;
      }
    }
    ReactDOM.render(<Outer><Component /></Outer>, container);
    ReactDOM.render(<Outer />, container);
  });
  it('should warn when mutated props are passed', () => {
    spyOn(console, 'error');
    var container = document.createElement('div');
    class Foo extends React.Component {