fixes RawDialogRoute memory leak (#147817)

part of #141198 

- Fixes memory leak on `RawDialogRoute`
- Adds opt-in test

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

@@ -2286,10 +2286,7 @@ class RawDialogRoute<T> extends PopupRoute<T> {
     if (_transitionBuilder == null) {
       // Some default transition.
       return FadeTransition(
-        opacity: CurvedAnimation(
-          parent: animation,
-          curve: Curves.linear,
-        ),
+        opacity: animation,
         child: child,
       );
     }

packages/flutter/test/widgets/routes_test.dart

@@ -8,6 +8,7 @@ import 'dart:ui';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 import 'semantics_tester.dart';
 
@@ -974,7 +975,10 @@ void main() {
       expect(secondaryAnimationOfRouteOne.value, primaryAnimationOfRouteTwo.value);
     });
 
-    testWidgets('showGeneralDialog handles transparent barrier color', (WidgetTester tester) async {
+    testWidgets('showGeneralDialog handles transparent barrier color',
+    // TODO(polina-c): remove when fixed https://github.com/flutter/flutter/issues/145600 [leak-tracking-opt-in]
+    experimentalLeakTesting: LeakTesting.settings.withTracked(classes: <String>['CurvedAnimation']),
+    (WidgetTester tester) async {
       await tester.pumpWidget(MaterialApp(
         home: Builder(
           builder: (BuildContext context) {