add static method to create material hero controller (#61662)

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

@@ -536,6 +536,17 @@ class MaterialApp extends StatefulWidget {
 
   @override
   _MaterialAppState createState() => _MaterialAppState();
+
+  /// The [HeroController] used for Material page transitions.
+  ///
+  /// Used by the [MaterialApp].
+  static HeroController createMaterialHeroController() {
+    return HeroController(
+      createRectTween: (Rect begin, Rect end) {
+        return MaterialRectArcTween(begin: begin, end: end);
+      },
+    );
+  }
 }
 
 class _MaterialScrollBehavior extends ScrollBehavior {
@@ -572,7 +583,7 @@ class _MaterialAppState extends State<MaterialApp> {
   @override
   void initState() {
     super.initState();
-    _heroController = HeroController(createRectTween: _createRectTween);
+    _heroController = MaterialApp.createMaterialHeroController();
   }
 
   @override
@@ -583,14 +594,10 @@ class _MaterialAppState extends State<MaterialApp> {
       // old Navigator won't be disposed (and thus won't unregister with its
       // observers) until after the new one has been created (because the
       // Navigator has a GlobalKey).
-      _heroController = HeroController(createRectTween: _createRectTween);
+      _heroController = MaterialApp.createMaterialHeroController();
     }
   }
 
-  RectTween _createRectTween(Rect begin, Rect end) {
-    return MaterialRectArcTween(begin: begin, end: end);
-  }
-
   // Combine the Localizations for Material with the ones contributed
   // by the localizationsDelegates parameter, if any. Only the first delegate
   // of a particular LocalizationsDelegate.type is loaded so the

packages/flutter/test/material/app_test.dart

@@ -834,6 +834,15 @@ void main() {
     expect(find.text('regular page one'), findsNothing);
     expect(find.text('regular page two'), findsNothing);
   });
+
+  testWidgets('MaterialApp does create HeroController with the MaterialRectArcTween', (WidgetTester tester) async {
+    final HeroController controller = MaterialApp.createMaterialHeroController();
+    final Tween<Rect> tween = controller.createRectTween(
+      const Rect.fromLTRB(0.0, 0.0, 10.0, 10.0),
+      const Rect.fromLTRB(0.0, 0.0, 20.0, 20.0)
+    );
+    expect(tween, isA<MaterialRectArcTween>());
+  });
 }
 
 class MockAccessibilityFeature implements AccessibilityFeatures {