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).
state = {value: 0};
      componentWillUnmount() {
        expect(() => {
          this.setState({value: 2}, function() {
            cbCalled = true;
          });
        }).not.toThrow();
      }
      render() {
        return <div />;
      }
    }
    var instance = ReactDOM.render(<Component />, container);
    instance.setState({value: 1});
    ReactDOM.unmountComponentAtNode(container);
    expect(cbCalled).toBe(false);
  });
  it('should warn about `setState` in render', () => {
    spyOn(console, 'error');
    var container = document.createElement('div');
    var renderedState = -1;
    var renderPasses = 0;
    class Component extends React.Component {