Fix heroes transition when initial route is not '/' (#24039)

* early return for initialRoute + test

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

@@ -598,9 +598,18 @@ class HeroController extends NavigatorObserver {
       final PageRoute<dynamic> to = toRoute;
       final Animation<double> animation = (flightType == HeroFlightDirection.push) ? to.animation : from.animation;
 
-      // A user gesture may have already completed the pop.
-      if (flightType == HeroFlightDirection.pop && animation.status == AnimationStatus.dismissed) {
-        return;
+      // A user gesture may have already completed the pop, or we might be the initial route
+      switch (flightType) {
+        case HeroFlightDirection.pop:
+          if (animation.value == 0.0) {
+            return;
+          }
+          break;
+        case HeroFlightDirection.push:
+          if (animation.value == 1.0) {
+            return;
+          }
+          break;
       }
 
       // For pop transitions driven by a user gesture: if the "to" page has

packages/flutter/test/widgets/heroes_test.dart

@@ -1437,4 +1437,12 @@ void main() {
     expect(find.byKey(firstKey), isInCard);
     expect(find.byKey(secondKey), findsNothing);
   });
+
+  testWidgets('Handles transitions when a non-default initial route is set', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      routes: routes,
+      initialRoute: '/two',
+    ));
+    expect(find.text('two'), findsOneWidget);
+  });
 }