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 contextTypes = {
        flag: PropTypes.bool,
      };
      render() {
        return <div />;
      }
    }
    class Wrapper extends React.Component {
      render() {
        return (
          <Parent ref="parent">
            <Child ref="child" />
          </Parent>
        );
      }
    }
    var wrapper = ReactTestUtils.renderIntoDocument(<Wrapper />);
    expect(wrapper.refs.parent.state.flag).toEqual(true);
    expect(wrapper.refs.child.context).toEqual({flag: true});
    
    wrapper.refs.parent.setState({flag: false});
    expect(wrapper.refs.parent.state.flag).toEqual(false);
    expect(wrapper.refs.child.context).toEqual({flag: false});
  });
  it('should pass context transitively', () => {
    var childInstance = null;
    var grandchildInstance = null;
    class Parent extends React.Component {