Add navigationContext to react-native Navigator

react-native\Libraries\CustomComponents\Navigator\Navigator.js

var Navigator = React.createClass({
  ...
  componentWillMount: function() {
    // TODO(t7489503): Don't need this once ES6 Class landed.
    this.__defineGetter__('navigationContext', this._getNavigationContext);

    this._subRouteFocus = [];
    ...
    ... 
}

react-native\Libraries\CustomComponents\Navigator\Navigation\NavigationContext.js

class NavigationContext {
   ...
}

react-native/react-native.d.ts

@@ -3247,6 +3247,19 @@ declare namespace  __React {
         debugOverlay?: boolean
 
     }
+    
+   /**
+   * Class that contains the info and methods for app navigation.
+   */
+    export interface NavigationContext { 
+        parent: NavigationContext,
+        top: NavigationContext,
+        currentRoute: any,
+        appendChild(childContext: NavigationContext): void;
+        addListener(eventType: string, listener: () => void, useCapture?: boolean): NativeEventSubscription;
+        emit(eventType: string, data: any, didEmitCallback?: () => void): void;
+        dispose(): void;
+    }
 
     /**
      * Use Navigator to transition between different scenes in your app.
@@ -3262,6 +3275,8 @@ declare namespace  __React {
         NavigationBar: NavigatorStatic.NavigationBarStatic;
         BreadcrumbNavigationBar: NavigatorStatic.BreadcrumbNavigationBarStatic
 
+	navigationContext: NavigationContext;
+
         getContext( self: any ): NavigatorStatic;
 
         /**