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).
var toggleActivatedState = this._toggleActivatedState;
        return !this.state.activated
          ? <a ref="x" onClick={toggleActivatedState} />
          : <b ref="x" onClick={toggleActivatedState} />;
      }
    };
    /**
     * We'll use this to ensure that an old version is not cached when it is
     * reallocated again.
     */
    ChildUpdates = class extends React.Component {
      getAnchor = () => {
        return this.refs.anch;
      };
      render() {
        var className = this.props.anchorClassOn ? 'anchorClass' : '';
        return this.props.renderAnchor
          ? <a ref="anch" className={className} />
          : <b />;
      }
    };
  });
  it('should support module pattern components', () => {
    function Child({test}) {