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="child-with">{this.props.children}</div>;
      }
    }
    class ChildWithoutContext extends React.Component {
      componentWillReceiveProps(nextProps, nextContext) {
        expect('foo' in nextContext).toBe(false);
        if (nextProps !== this.props) {
          propChanges++;
        }
        if (nextContext !== this.context) {
          contextChanges++;
        }
      }
      render() {
        return <div className="child-without">{this.props.children}</div>;
      }
    }
    class Parent extends React.Component {
      static childContextTypes = {
        foo: PropTypes.string,
      };
      state = {
        foo: 'abc',
      };
      getChildContext() {
        return {
          foo: this.state.foo,
        };
      }
      render() {