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);
      }
      shouldComponentUpdate(nextProps, nextState, nextContext) {
        expect('foo' in nextContext).toBe(true);
        return true;
      }
      render() {
        return <span>{this.context.foo}</span>;
      }
    }
    class Intermediary extends React.Component {
      componentWillReceiveProps(nextProps, nextContext) {
        expect('foo' in nextContext).toBe(false);
      }
      shouldComponentUpdate(nextProps, nextState, nextContext) {
        expect('foo' in nextContext).toBe(false);
        return true;
      }
      render() {
        return <Leaf />;
      }
    }
    class Parent extends React.Component {