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) {
      tryToClaimNextHydratableInstance(workInProgress);
    }
    let nextProps = workInProgress.pendingProps;
    if (nextProps === null) {
      nextProps = workInProgress.memoizedProps;
    }
    memoizeProps(workInProgress, nextProps);
    
    
    return null;
  }
  function mountIndeterminateComponent(
    current,
    workInProgress,
    renderExpirationTime,
  ) {
    invariant(
      current === null,
      'An indeterminate component should never have mounted. This error is ' +
        'likely caused by a bug in React. Please file an issue.',
    );
    var fn = workInProgress.type;
    var props = workInProgress.pendingProps;
    var unmaskedContext = getUnmaskedContext(workInProgress);
    var context = getMaskedContext(workInProgress, unmaskedContext);
    var value;
    if (__DEV__) {
      ReactCurrentOwner.current = workInProgress;
      value = fn(props, context);