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 childContextTypes = {
        foo: PropTypes.string,
        depth: PropTypes.number,
      };
      state = {
        flag: false,
      };
      getChildContext() {
        return {
          foo: 'bar',
          depth: 0,
        };
      }
      render() {
        var output = <Child />;
        if (!this.state.flag) {
          output = <span>Child</span>;
        }
        return output;
      }
    }
    class Child extends React.Component {