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 (workInProgress.ref !== null) {
          let info = '';
          const ownerName = ReactDebugCurrentFiber.getCurrentFiberOwnerName();
          if (ownerName) {
            info += '\n\nCheck the render method of `' + ownerName + '`.';
          }
          let warningKey = ownerName || workInProgress._debugID || '';
          const debugSource = workInProgress._debugSource;
          if (debugSource) {
            warningKey = debugSource.fileName + ':' + debugSource.lineNumber;
          }
          if (!warnedAboutStatelessRefs[warningKey]) {
            warnedAboutStatelessRefs[warningKey] = true;
            warning(
              false,
              'Stateless function components cannot be given refs. ' +
                'Attempts to access this ref will fail.%s%s',
              info,
              ReactDebugCurrentFiber.getCurrentFiberStackAddendum(),
            );
          }
        }
      }
      reconcileChildren(current, workInProgress, value);
      memoizeProps(workInProgress, props);
      return workInProgress.child;
    }
  }
  function updateCallComponent(current, workInProgress, renderExpirationTime) {