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).
} else {
      value = fn(props, context);
    }
    
    workInProgress.effectTag |= PerformedWork;
    if (
      typeof value === 'object' &&
      value !== null &&
      typeof value.render === 'function'
    ) {
      
      workInProgress.tag = ClassComponent;
      
      
      
      const hasContext = pushContextProvider(workInProgress);
      adoptClassInstance(workInProgress, value);
      mountClassInstance(workInProgress, renderExpirationTime);
      return finishClassComponent(current, workInProgress, true, hasContext);
    } else {
      
      workInProgress.tag = FunctionalComponent;
      if (__DEV__) {
        const Component = workInProgress.type;
        if (Component) {
          warning(
            !Component.childContextTypes,
            '%s(...): childContextTypes cannot be defined on a functional component.',
            Component.displayName || Component.name || 'Component',
          );