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).
static childContextTypes = {
        foo: PropTypes.string,
      };
      getChildContext() {
        return {
          foo: this.props.cntxt,
        };
      }
      render() {
        return <Intermediary />;
      }
    }
    var div = document.createElement('div');
    ReactDOM.render(<Parent cntxt="noise" />, div);
    expect(div.children[0].innerHTML).toBe('noise');
    div.children[0].innerHTML = 'aliens';
    div.children[0].id = 'aliens';
    expect(div.children[0].innerHTML).toBe('aliens');
    expect(div.children[0].id).toBe('aliens');
    ReactDOM.render(<Parent cntxt="bar" />, div);
    expect(div.children[0].innerHTML).toBe('bar');
    expect(div.children[0].id).toBe('aliens');
  });
  it('should trigger componentWillReceiveProps for context changes', () => {
    var contextChanges = 0;
    var propChanges = 0;
    class GrandChild extends React.Component {