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).
render() {
        return <Child><Grandchild /></Child>;
      }
    }
    class Child extends React.Component {
      static childContextTypes = {
        foo: PropTypes.string,
      };
      getChildContext() {
        return {
          foo: 'bar',
        };
      }
      render() {
        return React.Children.only(this.props.children);
      }
    }
    class Grandchild extends React.Component {
      static contextTypes = {
        foo: PropTypes.string,
      };
      render() {
        return <div>{this.context.foo}</div>;
      }
    }
    var component = ReactTestUtils.renderIntoDocument(<Parent />);
    expect(ReactDOM.findDOMNode(component).innerHTML).toBe('bar');
  });
  it('should skip update when rerendering element in container', () => {
    class Parent extends React.Component {