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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.

? 'getDerivedStateFromProps()'
            : 'getSnapshotBeforeUpdate()';
        if (!didWarnAboutLegacyLifecyclesAndDerivedState.has(componentName)) {
          didWarnAboutLegacyLifecyclesAndDerivedState.add(componentName);
          warningWithoutStack(
            false,
            'Unsafe legacy lifecycles will not be called for components using new component APIs.\n\n' +
              '%s uses %s but also contains the following legacy lifecycles:%s%s%s\n\n' +
              'The above lifecycles should be removed. Learn more about this warning here:\n' +
              'https://fb.me/react-async-component-lifecycle-hooks',
            componentName,
            newApiName,
            foundWillMountName !== null ? `\n  ${foundWillMountName}` : '',
            foundWillReceivePropsName !== null
              ? `\n  ${foundWillReceivePropsName}`
              : '',
            foundWillUpdateName !== null ? `\n  ${foundWillUpdateName}` : '',
          );
        }
      }
    }
  }
  
  
  if (isLegacyContextConsumer) {
    cacheContext(workInProgress, unmaskedContext, context);
  }
  return instance;
}
function callComponentWillMount(workInProgress, instance) {