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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.

it('should throw prop mapping error for an <iframe /> with invalid props', () => {
      let caughtErr;
      try {
        ReactDOMServer.renderToString(<iframe style="border:none;" />);
      } catch (err) {
        caughtErr = err;
      }
      expect(caughtErr).not.toBe(undefined);
      expect(normalizeCodeLocInfo(caughtErr.message)).toContain(
        'The `style` prop expects a mapping from style properties to values, not ' +
          "a string. For example, style={{marginRight: spacing + 'em'}} when using JSX." +
          (__DEV__ ? '\n    in iframe (at **)' : ''),
      );
    });
    it('should not crash on poisoned hasOwnProperty', () => {
      let html;
      expect(
        () =>
          (html = ReactDOMServer.renderToString(
            <div hasOwnProperty="poison">
              <span unknown="test" />
            </div>,
          )),
      ).toWarnDev(['React does not recognize the `hasOwnProperty` prop']);
      expect(html).toContain('<span unknown="test">');
    });
  });
  describe('renderToStaticMarkup', () => {
    it('should not put checksum and React ID on components', () => {
      class NestedComponent extends React.Component {