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).
let nextChildren = nextProps.children;
    const isDirectTextChild = shouldSetTextContent(type, nextProps);
    if (isDirectTextChild) {
      
      
      
      
      nextChildren = null;
    } else if (prevProps && shouldSetTextContent(type, prevProps)) {
      
      
      workInProgress.effectTag |= ContentReset;
    }
    markRef(current, workInProgress);
    
    if (
      renderExpirationTime !== Never &&
      !useSyncScheduling &&
      shouldDeprioritizeSubtree(type, nextProps)
    ) {
      
      workInProgress.expirationTime = Never;
      
      return null;
    }
    reconcileChildren(current, workInProgress, nextChildren);
    memoizeProps(workInProgress, nextProps);
    return workInProgress.child;
  }
  function updateHostText(current, workInProgress) {