Move the registration of the restoration channel to binding initialization. (#65579)

packages/flutter/lib/src/services/restoration.dart

@@ -97,6 +97,25 @@ typedef _BucketVisitor = void Function(RestorationBucket bucket);
 ///  * [RestorationMixin], which uses [RestorationBucket]s behind the scenes
 ///    to make [State] objects of [StatefulWidget]s restorable.
 class RestorationManager extends ChangeNotifier {
+  /// Construct the restoration manager and set up the communications channels
+  /// with the engine to get restoration messages (by calling [initChannels]).
+  RestorationManager() {
+    initChannels();
+  }
+
+  /// Sets up the method call handler for [SystemChannels.restoration].
+  ///
+  /// This is called by the constructor to configure the communications channel
+  /// with the Flutter engine to get restoration messages.
+  ///
+  /// Subclasses (especially in tests) can override this to avoid setting up
+  /// that communications channel, or to set it up differently, as necessary.
+  @protected
+  void initChannels() {
+    assert(!SystemChannels.restoration.checkMethodCallHandler(_methodHandler));
+    SystemChannels.restoration.setMethodCallHandler(_methodHandler);
+  }
+
   /// The root of the [RestorationBucket] hierarchy containing the restoration
   /// data.
   ///
@@ -128,9 +147,6 @@ class RestorationManager extends ChangeNotifier {
   ///  * [RootRestorationScope], which makes the root bucket available in the
   ///    [Widget] tree.
   Future<RestorationBucket?> get rootBucket {
-    if (!SystemChannels.restoration.checkMethodCallHandler(_methodHandler)) {
-      SystemChannels.restoration.setMethodCallHandler(_methodHandler);
-    }
     if (_rootBucketIsValid) {
       return SynchronousFuture<RestorationBucket?>(_rootBucket);
     }

packages/flutter/test/services/channel_buffers_test.dart

@@ -15,8 +15,7 @@ import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-class TestChannelBuffersFlutterBinding extends BindingBase with SchedulerBinding, ServicesBinding {
-}
+class TestChannelBuffersFlutterBinding extends BindingBase with SchedulerBinding, ServicesBinding { }
 
 void main() {
   ByteData _makeByteData(String str) {

packages/flutter/test/services/restoration.dart

@@ -10,11 +10,21 @@ import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 class MockRestorationManager extends TestRestorationManager {
+  MockRestorationManager({ this.enableChannels = false });
+
   bool get updateScheduled => _updateScheduled;
   bool _updateScheduled = false;
 
   final List<RestorationBucket> _buckets = <RestorationBucket>[];
 
+  final bool enableChannels;
+
+  @override
+  void initChannels() {
+    if (enableChannels)
+      super.initChannels();
+  }
+
   @override
   void scheduleSerializationFor(RestorationBucket bucket) {
     _updateScheduled = true;