Fixes the transition builder changing the Cupertino transition on Android. (#134790)

Fixes #124850.

Changes the fallback for the builder on Android. Before the error was from the platform being changed mid Cupertino transition to iOS. If a default builder wasn't set for iOS (which a developer developing only for Android might not), then it defaulted to the Zoom transition. Which changing the transition while on the fly caused issues.

packages/flutter/lib/src/material/page_transitions_theme.dart

@@ -771,8 +771,10 @@ class PageTransitionsTheme with Diagnosticable {
       platform = TargetPlatform.iOS;
     }
 
-    final PageTransitionsBuilder matchingBuilder =
-      builders[platform] ?? const ZoomPageTransitionsBuilder();
+    final PageTransitionsBuilder matchingBuilder = builders[platform] ?? switch (platform) {
+      TargetPlatform.iOS => const CupertinoPageTransitionsBuilder(),
+      TargetPlatform.android || TargetPlatform.fuchsia || TargetPlatform.windows || TargetPlatform.macOS || TargetPlatform.linux => const ZoomPageTransitionsBuilder(),
+    };
     return matchingBuilder.buildTransitions<T>(route, context, animation, secondaryAnimation, child);
   }
 

packages/flutter/test/material/page_transitions_theme_test.dart

@@ -134,6 +134,86 @@ void main() {
     expect(findOpenUpwardsPageTransition(), findsOneWidget);
   }, variant: TargetPlatformVariant.only(TargetPlatform.android));
 
+  testWidgets('PageTransitionsTheme override builds a CupertinoPageTransition on android', (WidgetTester tester) async {
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+      '/': (BuildContext context) => Material(
+        child: TextButton(
+          child: const Text('push'),
+          onPressed: () { Navigator.of(context).pushNamed('/b'); },
+        ),
+      ),
+      '/b': (BuildContext context) => const Text('page b'),
+    };
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          pageTransitionsTheme: const PageTransitionsTheme(
+            builders: <TargetPlatform, PageTransitionsBuilder>{
+              TargetPlatform.android: CupertinoPageTransitionsBuilder(),
+            },
+          ),
+        ),
+        routes: routes,
+      ),
+    );
+
+    expect(Theme.of(tester.element(find.text('push'))).platform, debugDefaultTargetPlatformOverride);
+    expect(find.byType(CupertinoPageTransition), findsOneWidget);
+
+    await tester.tap(find.text('push'));
+    await tester.pumpAndSettle();
+    expect(find.text('page b'), findsOneWidget);
+    expect(find.byType(CupertinoPageTransition), findsOneWidget);
+  }, variant: TargetPlatformVariant.only(TargetPlatform.android));
+
+  testWidgets('CupertinoPageTransition on android does not block gestures on backswipe', (WidgetTester tester) async {
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+      '/': (BuildContext context) => Material(
+        child: TextButton(
+          child: const Text('push'),
+          onPressed: () { Navigator.of(context).pushNamed('/b'); },
+        ),
+      ),
+      '/b': (BuildContext context) => const Text('page b'),
+    };
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          pageTransitionsTheme: const PageTransitionsTheme(
+            builders: <TargetPlatform, PageTransitionsBuilder>{
+              TargetPlatform.android: CupertinoPageTransitionsBuilder(),
+            },
+          ),
+        ),
+        routes: routes,
+      ),
+    );
+
+    expect(Theme.of(tester.element(find.text('push'))).platform, debugDefaultTargetPlatformOverride);
+    expect(find.byType(CupertinoPageTransition), findsOneWidget);
+
+    await tester.tap(find.text('push'));
+    await tester.pumpAndSettle();
+    expect(find.text('page b'), findsOneWidget);
+    expect(find.byType(CupertinoPageTransition), findsOneWidget);
+
+    await tester.pumpAndSettle(const Duration(minutes: 1));
+
+    final TestGesture gesture = await tester.startGesture(const Offset(5.0, 100.0));
+    await gesture.moveBy(const Offset(400.0, 0.0));
+    await gesture.up();
+    await tester.pump();
+
+    await tester.pumpAndSettle(const Duration(minutes: 1));
+
+    expect(find.text('push'), findsOneWidget);
+    await tester.tap(find.text('push'));
+    await tester.pumpAndSettle();
+    expect(find.text('page b'), findsOneWidget);
+  }, variant: TargetPlatformVariant.only(TargetPlatform.android));
+
   testWidgets('PageTransitionsTheme override builds a _FadeUpwardsTransition', (WidgetTester tester) async {
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
       '/': (BuildContext context) => Material(