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**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
Crash during server render due to `this.context` being undefined. Bug only happens in SSR, and only in dev mode react. Issue appears neither in browser renders, nor in production mode server renders.

note: Using https://reactjs.net/ for c# server-side rendering, with bundled-in version of react disabled (so using the same version of react as the client bundle)

(simplified) application structure
```
// root component
const {Provider, Consumer} = createContext({});
const Root = props => return <Provider value={props.notNull}>{props.children}</Provider>;

// consumer
class ConsumingComponent extends Component {
  static contextType = Consumer;

  render() {
    if (this.context.someValue) return null;
    return <div/>;
  }
}

// render tree
<Root>...<ConsumingComponent/>...</Root>

// output of server render
JsRuntimeException: TypeError: unable to get property 'someValue' of undefined or null reference.
```

**What is the expected behavior?**
`this.context` in the consuming component should be the value passed to the Provider, and not `undefined`.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

The bug, as best as I can pin down, appeared in 16.6.3. The code works in 16.6..1, and 16.6.2 is listed as a broken release in the changelog.

'getInitialState was defined on %s, a plain JavaScript class. ' +
        'This is only supported for classes created using React.createClass. ' +
        'Did you mean to define a state property instead?',
      name,
    );
    const noGetDefaultPropsOnES6 =
      !instance.getDefaultProps ||
      instance.getDefaultProps.isReactClassApproved;
    warningWithoutStack(
      noGetDefaultPropsOnES6,
      'getDefaultProps was defined on %s, a plain JavaScript class. ' +
        'This is only supported for classes created using React.createClass. ' +
        'Use a static property to define defaultProps instead.',
      name,
    );
    const noInstancePropTypes = !instance.propTypes;
    warningWithoutStack(
      noInstancePropTypes,
      'propTypes was defined as an instance property on %s. Use a static ' +
        'property to define propTypes instead.',
      name,
    );
    const noInstanceContextType = !instance.contextType;
    warningWithoutStack(
      noInstanceContextType,
      'contextType was defined as an instance property on %s. Use a static ' +
        'property to define contextType instead.',
      name,
    );
    const noInstanceContextTypes = !instance.contextTypes;