Add NestedBackButtonDispatcher

lib/src/advanced_navigator.dart

@@ -2,6 +2,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
 part 'route_information_provider.dart';
+part 'back_button_dispatcher.dart';
 
 /// A function for building a page stack from route information
 typedef PathFactory = RouterConfiguration Function(RouteInformation);

lib/src/back_button_dispatcher.dart

@@ -0,0 +1,24 @@
+part of 'advanced_navigator.dart';
+
+class NestedBackButtonDispatcher extends ChildBackButtonDispatcher {
+  NestedBackButtonDispatcher(
+    BackButtonDispatcher parent, {
+    @required Route route,
+  }) : _route = route,
+       super(parent);
+
+  final Route _route;
+
+  @override
+  Future<bool> invokeCallback(Future<bool> defaultValue) {
+    // only pop if navigator is contained in top-most route
+    if (_route.isCurrent) {
+      return super.invokeCallback(defaultValue);
+    } else {
+      parent.forget(this);
+      var value = parent.invokeCallback(defaultValue);
+      parent.deferTo(this);
+      return value;
+    }
+  }
+}