Refactor SliverFillRemaining (#47379)

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

@@ -22,7 +22,7 @@ import 'scroll_metrics.dart';
 import 'scroll_physics.dart';
 import 'scroll_position.dart';
 import 'scroll_view.dart';
-import 'sliver.dart';
+import 'sliver_fill.dart';
 import 'ticker_provider.dart';
 import 'viewport.dart';
 

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

@@ -25,6 +25,7 @@ import 'scroll_position_with_single_context.dart';
 import 'scroll_view.dart';
 import 'scrollable.dart';
 import 'sliver.dart';
+import 'sliver_fill.dart';
 import 'viewport.dart';
 
 /// A controller for [PageView].

packages/flutter/lib/widgets.dart

@@ -95,6 +95,7 @@ export 'src/widgets/shortcuts.dart';
 export 'src/widgets/single_child_scroll_view.dart';
 export 'src/widgets/size_changed_layout_notifier.dart';
 export 'src/widgets/sliver.dart';
+export 'src/widgets/sliver_fill.dart';
 export 'src/widgets/sliver_layout_builder.dart';
 export 'src/widgets/sliver_persistent_header.dart';
 export 'src/widgets/sliver_prototype_extent_list.dart';