Clean up scrolling delegate code for 2D (#122651)

Clean up scrolling delegate code for 2D

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

@@ -7,6 +7,7 @@ import 'package:flutter/foundation.dart';
 import 'basic.dart';
 import 'framework.dart';
 import 'scroll_controller.dart';
+import 'scroll_delegate.dart';
 import 'scroll_physics.dart';
 import 'scroll_view.dart';
 import 'sliver.dart';

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

@@ -16,6 +16,7 @@ import 'page_storage.dart';
 import 'scroll_configuration.dart';
 import 'scroll_context.dart';
 import 'scroll_controller.dart';
+import 'scroll_delegate.dart';
 import 'scroll_metrics.dart';
 import 'scroll_notification.dart';
 import 'scroll_physics.dart';
@@ -23,7 +24,6 @@ import 'scroll_position.dart';
 import 'scroll_position_with_single_context.dart';
 import 'scroll_view.dart';
 import 'scrollable.dart';
-import 'sliver.dart';
 import 'sliver_fill.dart';
 import 'viewport.dart';
 

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

@@ -13,6 +13,7 @@ import 'inherited_theme.dart';
 import 'media_query.dart';
 import 'overlay.dart';
 import 'scroll_controller.dart';
+import 'scroll_delegate.dart';
 import 'scroll_physics.dart';
 import 'scroll_view.dart';
 import 'scrollable.dart';

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

@@ -17,6 +17,7 @@ import 'notification_listener.dart';
 import 'primary_scroll_controller.dart';
 import 'scroll_configuration.dart';
 import 'scroll_controller.dart';
+import 'scroll_delegate.dart';
 import 'scroll_notification.dart';
 import 'scroll_physics.dart';
 import 'scrollable.dart';

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

@@ -6,6 +6,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 
 import 'framework.dart';
+import 'scroll_delegate.dart';
 import 'sliver.dart';
 
 /// A sliver that contains multiple box children that each fills the viewport.

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

@@ -5,6 +5,7 @@
 import 'package:flutter/rendering.dart';
 
 import 'framework.dart';
+import 'scroll_delegate.dart';
 import 'sliver.dart';
 
 /// A sliver that places its box children in a linear array and constrains them

packages/flutter/lib/widgets.dart

@@ -106,6 +106,7 @@ export 'src/widgets/scroll_aware_image_provider.dart';
 export 'src/widgets/scroll_configuration.dart';
 export 'src/widgets/scroll_context.dart';
 export 'src/widgets/scroll_controller.dart';
+export 'src/widgets/scroll_delegate.dart';
 export 'src/widgets/scroll_metrics.dart';
 export 'src/widgets/scroll_notification.dart';
 export 'src/widgets/scroll_notification_observer.dart';