Fix nullability warnings in routes_test (#67694)

packages/flutter/test/widgets/routes_test.dart

@@ -57,7 +57,7 @@ class TestRoute extends Route<String?> with LocalHistoryRoute<String?> {
   @override
   void didReplace(Route<dynamic>? oldRoute) {
     expect(oldRoute, isA<TestRoute>());
-    final TestRoute castRoute = oldRoute as TestRoute;
+    final TestRoute castRoute = oldRoute! as TestRoute;
     log('didReplace ${castRoute.name}');
     super.didReplace(castRoute);
   }
@@ -667,7 +667,7 @@ void main() {
       );
       await tester.pump();
       await tester.pumpAndSettle();
-      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent as ProxyAnimation;
+      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent! as ProxyAnimation;
       expect(animationPageOne.value, 1.0);
       expect(secondaryAnimationPageOne.parent, kAlwaysDismissedAnimation);
 
@@ -686,7 +686,7 @@ void main() {
       );
       await tester.pump();
       await tester.pumpAndSettle();
-      final ProxyAnimation secondaryAnimationPageTwo = secondaryAnimationProxyPageTwo.parent as ProxyAnimation;
+      final ProxyAnimation secondaryAnimationPageTwo = secondaryAnimationProxyPageTwo.parent! as ProxyAnimation;
       expect(animationPageTwo.value, 1.0);
       expect(secondaryAnimationPageTwo.parent, kAlwaysDismissedAnimation);
       expect(secondaryAnimationPageOne.parent, animationPageTwo.parent);
@@ -725,7 +725,7 @@ void main() {
       );
       await tester.pump();
       await tester.pumpAndSettle();
-      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent as ProxyAnimation;
+      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent! as ProxyAnimation;
       expect(animationPageOne.value, 1.0);
       expect(secondaryAnimationPageOne.parent, kAlwaysDismissedAnimation);
 
@@ -745,7 +745,7 @@ void main() {
       );
       await tester.pump();
       await tester.pumpAndSettle();
-      final ProxyAnimation secondaryAnimationPageTwo = secondaryAnimationProxyPageTwo.parent as ProxyAnimation;
+      final ProxyAnimation secondaryAnimationPageTwo = secondaryAnimationProxyPageTwo.parent! as ProxyAnimation;
       expect(animationPageTwo.value, 1.0);
       expect(secondaryAnimationPageTwo.parent, kAlwaysDismissedAnimation);
       expect(secondaryAnimationPageOne.parent, animationPageTwo.parent);
@@ -780,7 +780,7 @@ void main() {
       );
       await tester.pump();
       await tester.pumpAndSettle();
-      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent as ProxyAnimation;
+      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent! as ProxyAnimation;
       expect(animationPageOne.value, 1.0);
       expect(secondaryAnimationPageOne.parent, kAlwaysDismissedAnimation);
 
@@ -813,7 +813,7 @@ void main() {
       await tester.pump();
       await tester.pump(const Duration(milliseconds: 1));
       expect(secondaryAnimationPageOne.parent, isA<TrainHoppingAnimation>());
-      final TrainHoppingAnimation trainHopper = secondaryAnimationPageOne.parent as TrainHoppingAnimation;
+      final TrainHoppingAnimation trainHopper = secondaryAnimationPageOne.parent! as TrainHoppingAnimation;
       expect(trainHopper.currentTrain, animationPageTwo.parent);
       await tester.pump(const Duration(milliseconds: 100));
       expect(secondaryAnimationPageOne.parent, isNot(isA<TrainHoppingAnimation>()));
@@ -852,7 +852,7 @@ void main() {
       );
       await tester.pump();
       await tester.pumpAndSettle();
-      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent as ProxyAnimation;
+      final ProxyAnimation secondaryAnimationPageOne = secondaryAnimationProxyPageOne.parent! as ProxyAnimation;
       expect(animationPageOne.value, 1.0);
       expect(secondaryAnimationPageOne.parent, kAlwaysDismissedAnimation);
 
@@ -883,14 +883,14 @@ void main() {
       await tester.pump();
       await tester.pump(const Duration(milliseconds: 10));
       expect(secondaryAnimationPageOne.parent, isA<TrainHoppingAnimation>());
-      final TrainHoppingAnimation trainHopper = secondaryAnimationPageOne.parent as TrainHoppingAnimation;
+      final TrainHoppingAnimation trainHopper = secondaryAnimationPageOne.parent! as TrainHoppingAnimation;
       expect(trainHopper.currentTrain, animationPageTwo.parent);
 
       // Pop page three while replacement push is ongoing.
       navigator.currentState!.pop();
       await tester.pump();
       expect(secondaryAnimationPageOne.parent, isA<TrainHoppingAnimation>());
-      final TrainHoppingAnimation trainHopper2 = secondaryAnimationPageOne.parent as TrainHoppingAnimation;
+      final TrainHoppingAnimation trainHopper2 = secondaryAnimationPageOne.parent! as TrainHoppingAnimation;
       expect(trainHopper2.currentTrain, animationPageTwo.parent);
       expect(trainHopper.currentTrain, isNull); // Has been disposed.
       await tester.pumpAndSettle();