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).
if (current === null) {
      workInProgress.stateNode = mountChildFibersInPlace(
        workInProgress,
        workInProgress.stateNode,
        nextChildren,
        renderExpirationTime,
      );
    } else if (current.child === workInProgress.child) {
      workInProgress.stateNode = reconcileChildFibers(
        workInProgress,
        workInProgress.stateNode,
        nextChildren,
        renderExpirationTime,
      );
    } else {
      workInProgress.stateNode = reconcileChildFibersInPlace(
        workInProgress,
        workInProgress.stateNode,
        nextChildren,
        renderExpirationTime,
      );
    }
    memoizeProps(workInProgress, nextCall);
    
    
    return workInProgress.stateNode;
  }
  function updatePortalComponent(