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).
case CallComponent:
        return updateCallComponent(
          current,
          workInProgress,
          renderExpirationTime,
        );
      case ReturnComponent:
        
        
        return null;
      case HostPortal:
        return updatePortalComponent(
          current,
          workInProgress,
          renderExpirationTime,
        );
      case Fragment:
        return updateFragment(current, workInProgress);
      default:
        invariant(
          false,
          'Unknown unit of work tag. This error is likely caused by a bug in ' +
            'React. Please file an issue.',
        );
    }
  }
  function beginFailedWork(
    current: Fiber | null,
    workInProgress: Fiber,