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).
shouldComponentUpdate() {
        return false;
      }
      render() {
        return <div>{this.props.children}</div>;
      }
    }
    class Component extends React.Component {
      render() {
        if (this.props.flipped) {
          return (
            <div>
              <Static ref="static0" key="B">B (ignored)</Static>
              <Static ref="static1" key="A">A (ignored)</Static>
            </div>
          );
        } else {
          return (
            <div>
              <Static ref="static0" key="A">A</Static>
              <Static ref="static1" key="B">B</Static>
            </div>
          );
        }
      }
    }
    var container = document.createElement('div');
    var comp = ReactDOM.render(<Component flipped={false} />, container);
    expect(ReactDOM.findDOMNode(comp.refs.static0).textContent).toBe('A');
    expect(ReactDOM.findDOMNode(comp.refs.static1).textContent).toBe('B');