Add method stopPropagation and stop to NavigationEvent

Summary: @​public

Add method `stopPropagation` and `stop` to NavigationEvent so we can stop event easily once event bubbling and capturing is supported.

Reviewed By: @fkgozali

Differential Revision: D2471903

Libraries/CustomComponents/Navigator/Navigation/NavigationEvent.js

@@ -57,6 +57,7 @@ var _navigationEventPool = new NavigationEventPool();
 class NavigationEvent {
   _data: any;
   _defaultPrevented: boolean;
+  _propagationStopped: boolean;
   _disposed: boolean;
   _target: ?Object;
   _type: ?string;
@@ -71,6 +72,7 @@ class NavigationEvent {
     this._data = data;
     this._defaultPrevented = false;
     this._disposed = false;
+    this._propagationStopped = false;
   }
 
   /* $FlowFixMe - get/set properties not yet supported */
@@ -97,6 +99,19 @@ class NavigationEvent {
     this._defaultPrevented = true;
   }
 
+  stopPropagation(): void {
+    this._propagationStopped = true;
+  }
+
+  stop(): void {
+    this.preventDefault();
+    this.stopPropagation();
+  }
+
+  isPropagationStopped(): boolean {
+    return this._propagationStopped;
+  }
+
   /**
    * Dispose the event.
    * NavigationEvent shall be disposed after being emitted by