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 = {
        foo: PropTypes.string.isRequired,
      };
      componentWillReceiveProps(nextProps, nextContext) {
        expect('foo' in nextContext).toBe(true);
        if (nextProps !== this.props) {
          propChanges++;
        }
        if (nextContext !== this.context) {
          contextChanges++;
        }
      }
      render() {
        return <span className="grand-child">{this.props.children}</span>;
      }
    }
    class ChildWithContext extends React.Component {
      static contextTypes = {
        foo: PropTypes.string.isRequired,
      };
      componentWillReceiveProps(nextProps, nextContext) {
        expect('foo' in nextContext).toBe(true);
        if (nextProps !== this.props) {
          propChanges++;
        }
        if (nextContext !== this.context) {
          contextChanges++;
        }
      }
      render() {