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).
var ReactDOM;
var ReactDOMServer;
var ReactCurrentOwner;
var ReactTestUtils;
var PropTypes;
var shallowEqual;
var shallowCompare;
describe('ReactCompositeComponent', () => {
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
    ReactDOMServer = require('react-dom/server');
    ReactCurrentOwner = require('react')
      .__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.ReactCurrentOwner;
    ReactTestUtils = require('react-dom/test-utils');
    PropTypes = require('prop-types');
    shallowEqual = require('fbjs/lib/shallowEqual');
    shallowCompare = function(instance, nextProps, nextState) {
      return (
        !shallowEqual(instance.props, nextProps) ||
        !shallowEqual(instance.state, nextState)
      );
    };
    MorphingComponent = class extends React.Component {
      state = {activated: false};
      _toggleActivatedState = () => {
        this.setState({activated: !this.state.activated});
      };
      render() {