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).
<ChildUpdates renderAnchor={false} anchorClassOn={true} />,
      container,
    );
    ReactDOM.render(
      
      <ChildUpdates renderAnchor={true} anchorClassOn={false} />,
      container,
    );
    expect(instance.getAnchor().className).toBe('');
  });
  it('should use default values for undefined props', () => {
    class Component extends React.Component {
      static defaultProps = {prop: 'testKey'};
      render() {
        return <span />;
      }
    }
    var instance1 = ReactTestUtils.renderIntoDocument(<Component />);
    expect(instance1.props).toEqual({prop: 'testKey'});
    var instance2 = ReactTestUtils.renderIntoDocument(
      <Component prop={undefined} />,
    );
    expect(instance2.props).toEqual({prop: 'testKey'});
    var instance3 = ReactTestUtils.renderIntoDocument(
      <Component prop={null} />,
    );
    expect(instance3.props).toEqual({prop: null});
  });
  it('should not mutate passed-in props object', () => {
    class Component extends React.Component {