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).
return <div className="parent">{this.props.children}</div>;
      }
    }
    var div = document.createElement('div');
    var parentInstance = null;
    ReactDOM.render(
      <Parent ref={inst => (parentInstance = inst)}>
        <ChildWithoutContext>
          A1
          <GrandChild>A2</GrandChild>
        </ChildWithoutContext>
        <ChildWithContext>
          B1
          <GrandChild>B2</GrandChild>
        </ChildWithContext>
      </Parent>,
      div,
    );
    parentInstance.setState({
      foo: 'def',
    });
    expect(propChanges).toBe(0);
    expect(contextChanges).toBe(3); 
  });
  it('should disallow nested render calls', () => {
    spyOn(console, 'error');
    class Inner extends React.Component {