make router assert more strict (#67672)

packages/flutter/lib/src/widgets/router.dart

@@ -245,7 +245,12 @@ class Router<T> extends StatefulWidget {
     this.routeInformationParser,
     required this.routerDelegate,
     this.backButtonDispatcher,
-  })  : assert(routeInformationProvider == null || routeInformationParser != null),
+  })  : assert(
+          (routeInformationProvider == null) == (routeInformationParser == null),
+          'You must provide both routeInformationProvider and routeInformationParser '
+          'if this router parses route information. Otheriwse, they should both '
+          'be null.'
+        ),
         assert(routerDelegate != null),
         super(key: key);
 

packages/flutter/test/widgets/router_test.dart

@@ -118,6 +118,50 @@ void main() {
     expect(find.text('popped'), findsOneWidget);
   });
 
+  testWidgets('Router throw when passes only routeInformationProvider', (WidgetTester tester) async {
+    final SimpleRouteInformationProvider provider = SimpleRouteInformationProvider();
+    provider.value = const RouteInformation(
+      location: 'initial',
+    );
+    try {
+      Router<RouteInformation>(
+        routeInformationProvider: provider,
+        routerDelegate: SimpleRouterDelegate(
+          builder: (BuildContext context, RouteInformation information) {
+            return Text(information.location);
+          },
+        ),
+      );
+    } on AssertionError catch(e) {
+      expect(
+        e.message,
+        'You must provide both routeInformationProvider and '
+        'routeInformationParser if this router parses route information. '
+        'Otheriwse, they should both be null.'
+      );
+    }
+  });
+
+  testWidgets('Router throw when passes only routeInformationParser', (WidgetTester tester) async {
+    try {
+      Router<RouteInformation>(
+        routeInformationParser: SimpleRouteInformationParser(),
+        routerDelegate: SimpleRouterDelegate(
+          builder: (BuildContext context, RouteInformation information) {
+            return Text(information.location);
+          },
+        ),
+      );
+    } on AssertionError catch(e) {
+      expect(
+        e.message,
+        'You must provide both routeInformationProvider and '
+        'routeInformationParser if this router parses route information. '
+        'Otheriwse, they should both be null.'
+      );
+    }
+  });
+
   testWidgets('PopNavigatorRouterDelegateMixin works', (WidgetTester tester) async {
     final SimpleRouteInformationProvider provider = SimpleRouteInformationProvider();
     provider.value = const RouteInformation(