Re-land fix for not disposed TabController (#146745)

Fixes flutter/flutter#144910

Author: polina-c

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

@@ -140,13 +140,15 @@ class TabController extends ChangeNotifier {
 
 
   /// Creates a new [TabController] with `index`, `previousIndex`, `length`, and
-  /// `animationDuration` if they are non-null.
+  /// `animationDuration` if they are non-null, and disposes current instance.
   ///
   /// This method is used by [DefaultTabController].
   ///
   /// When [DefaultTabController.length] is updated, this method is called to
   /// create a new [TabController] without creating a new [AnimationController].
-  TabController _copyWith({
+  /// Instead the [_animationController] is nulled in current instance and
+  /// passed to the new instance.
+  TabController _copyWithAndDispose({
     required int? index,
     required int? length,
     required int? previousIndex,
@@ -155,13 +157,16 @@ class TabController extends ChangeNotifier {
     if (index != null) {
       _animationController!.value = index.toDouble();
     }
-    return TabController._(
+    final TabController result = TabController._(
       index: index ?? _index,
       length: length ?? this.length,
       animationController: _animationController,
       previousIndex: previousIndex ?? _previousIndex,
       animationDuration: animationDuration ?? _animationDuration,
     );
+    _animationController = null;
+    dispose();
+    return result;
   }
 
   /// An animation whose value represents the current position of the [TabBar]'s
@@ -489,7 +494,7 @@ class _DefaultTabControllerState extends State<DefaultTabController> with Single
         newIndex = math.max(0, widget.length - 1);
         previousIndex = _controller.index;
       }
-      _controller = _controller._copyWith(
+      _controller = _controller._copyWithAndDispose(
         length: widget.length,
         animationDuration: widget.animationDuration,
         index: newIndex,
@@ -498,7 +503,7 @@ class _DefaultTabControllerState extends State<DefaultTabController> with Single
     }
 
     if (oldWidget.animationDuration != widget.animationDuration) {
-      _controller = _controller._copyWith(
+      _controller = _controller._copyWithAndDispose(
         length: widget.length,
         animationDuration: widget.animationDuration,
         index: _controller.index,

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

@@ -126,6 +126,9 @@ class _AnimatedState extends State<AnimatedWidget> {
   }
 
   void _handleChange() {
+    if (!mounted) {
+      return;
+    }
     setState(() {
       // The listenable's state is our build state, and it changed already.
     });

packages/flutter/test/material/tabs_test.dart

@@ -7,7 +7,6 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.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 '../widgets/semantics_tester.dart';
 import 'feedback_tester.dart';
@@ -121,9 +120,6 @@ Widget buildLeftRightApp({required List<String> tabs, required String value, boo
 }
 
 void main() {
-  // TODO(polina-c): dispose TabController, https://github.com/flutter/flutter/issues/144910 [leaks-to-clean]
-  LeakTesting.settings = LeakTesting.settings.withIgnoredAll();
-
   setUp(() {
     debugResetSemanticsIdCounter();
   });