fixes DialogRoute memory leak (#147816)

Author: Dimilkalathiya

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

@@ -1308,13 +1308,7 @@ class SimpleDialog extends StatelessWidget {
 }
 
 Widget _buildMaterialDialogTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
-  return FadeTransition(
-    opacity: CurvedAnimation(
-      parent: animation,
-      curve: Curves.easeOut,
-    ),
-    child: child,
-  );
+  return child;
 }
 
 /// Displays a Material dialog above the current contents of the app, with
@@ -1589,6 +1583,33 @@ class DialogRoute<T> extends RawDialogRoute<T> {
          transitionDuration: const Duration(milliseconds: 150),
          transitionBuilder: _buildMaterialDialogTransitions,
        );
+
+  CurvedAnimation? _curvedAnimation;
+
+  void _setAnimation(Animation<double> animation) {
+    if (_curvedAnimation?.parent != animation) {
+      _curvedAnimation?.dispose();
+      _curvedAnimation = CurvedAnimation(
+      parent: animation,
+      curve: Curves.easeOut,
+      );
+    }
+  }
+
+  @override
+  Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
+    _setAnimation(animation);
+    return FadeTransition(
+      opacity: _curvedAnimation!,
+      child: super.buildTransitions(context, animation, secondaryAnimation, child),
+    );
+  }
+
+  @override
+  void dispose() {
+    _curvedAnimation?.dispose();
+    super.dispose();
+  }
 }
 
 double _scalePadding(double textScaleFactor) {

packages/flutter/test/cupertino/dialog_test.dart

@@ -15,6 +15,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 import '../widgets/semantics_tester.dart';
 
@@ -355,7 +356,10 @@ void main() {
     expect(tester.getSize(find.widgetWithText(CupertinoDialogAction, 'OK')), equals(const Size(310.0, 98.0)));
   });
 
-  testWidgets('Dialog respects small constraints.', (WidgetTester tester) async {
+  testWidgets('Dialog respects small constraints.',
+  // TODO(polina-c): remove when fixed https://github.com/flutter/flutter/issues/145600 [leak-tracking-opt-in]
+  experimentalLeakTesting: LeakTesting.settings.withTracked(classes: const <String>['CurvedAnimation']),
+  (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     addTearDown(scrollController.dispose);
     await tester.pumpWidget(