Light sliver clean up before SliverTree (flutter#146696)

This cleans up a few sliver classes, like moving RenderSliverVariedExtentList to the rendering layer (it was in the widgets layer), and moving SliverVariedExtentList to live with its sibling subclasses, SliverFixedExtentList, SliverList, and so on.
I moved these while working on SliverTree, so figure I should break out into a separate change.

SliverTree and SliverCarousel (both inbound in separate changes) will also be subclasses of RenderSliverFixedExtentBoxAdaptor, organizing them together felt easier to work with.

Related to flutter#114299 and flutter#125980

packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart

@@ -466,3 +466,28 @@ class RenderSliverFixedExtentList extends RenderSliverFixedExtentBoxAdaptor {
     markNeedsLayout();
   }
 }
+
+/// A sliver that places multiple box children with the corresponding main axis extent in
+/// a linear array.
+class RenderSliverVariedExtentList extends RenderSliverFixedExtentBoxAdaptor {
+  /// Creates a sliver that contains multiple box children that have a explicit
+  /// extent in the main axis.
+  RenderSliverVariedExtentList({
+    required super.childManager,
+    required ItemExtentBuilder itemExtentBuilder,
+  }) : _itemExtentBuilder = itemExtentBuilder;
+
+  @override
+  ItemExtentBuilder get itemExtentBuilder => _itemExtentBuilder;
+  ItemExtentBuilder _itemExtentBuilder;
+  set itemExtentBuilder(ItemExtentBuilder value) {
+    if (_itemExtentBuilder == value) {
+      return;
+    }
+    _itemExtentBuilder = value;
+    markNeedsLayout();
+  }
+
+  @override
+  double? get itemExtent => null;
+}

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

@@ -22,7 +22,6 @@ import 'scrollable.dart';
 import 'scrollable_helpers.dart';
 import 'sliver.dart';
 import 'sliver_prototype_extent_list.dart';
-import 'sliver_varied_extent_list.dart';
 import 'ticker_provider.dart';
 import 'transitions.dart';
 

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

@@ -24,7 +24,6 @@ import 'scrollable.dart';
 import 'scrollable_helpers.dart';
 import 'sliver.dart';
 import 'sliver_prototype_extent_list.dart';
-import 'sliver_varied_extent_list.dart';
 import 'viewport.dart';
 
 // Examples can assume:

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

@@ -521,6 +521,113 @@ class SliverFixedExtentList extends SliverMultiBoxAdaptorWidget {
   }
 }
 
+/// A sliver that places its box children in a linear array and constrains them
+/// to have the corresponding extent returned by [itemExtentBuilder].
+///
+/// _To learn more about slivers, see [CustomScrollView.slivers]._
+///
+/// [SliverVariedExtentList] arranges its children in a line along
+/// the main axis starting at offset zero and without gaps. Each child is
+/// constrained to the corresponding extent along the main axis
+/// and the [SliverConstraints.crossAxisExtent] along the cross axis.
+///
+/// [SliverVariedExtentList] is more efficient than [SliverList] because
+/// [SliverVariedExtentList] does not need to lay out its children to obtain
+/// their extent along the main axis. It's a little more flexible than
+/// [SliverFixedExtentList] because this allow the children to have different extents.
+///
+/// See also:
+///
+///  * [SliverFixedExtentList], whose children are forced to a given pixel
+///    extent.
+///  * [SliverPrototypeExtentList], which is similar to [SliverFixedExtentList]
+///    except that it uses a prototype list item instead of a pixel value to define
+///    the main axis extent of each item.
+///  * [SliverList], which does not require its children to have the same
+///    extent in the main axis.
+///  * [SliverFillViewport], which sizes its children based on the
+///    size of the viewport, regardless of what else is in the scroll view.
+class SliverVariedExtentList extends SliverMultiBoxAdaptorWidget {
+  /// Creates a sliver that places box children with the same main axis extent
+  /// in a linear array.
+  const SliverVariedExtentList({
+    super.key,
+    required super.delegate,
+    required this.itemExtentBuilder,
+  });
+
+  /// A sliver that places multiple box children in a linear array along the main
+  /// axis.
+  ///
+  /// [SliverVariedExtentList] places its children in a linear array along the main
+  /// axis starting at offset zero and without gaps. Each child is forced to have
+  /// the returned extent of [itemExtentBuilder] in the main axis and the
+  /// [SliverConstraints.crossAxisExtent] in the cross axis.
+  ///
+  /// This constructor is appropriate for sliver lists with a large (or
+  /// infinite) number of children whose extent is already determined.
+  ///
+  /// Providing a non-null `itemCount` improves the ability of the [SliverGrid]
+  /// to estimate the maximum scroll extent.
+  SliverVariedExtentList.builder({
+    super.key,
+    required NullableIndexedWidgetBuilder itemBuilder,
+    required this.itemExtentBuilder,
+    ChildIndexGetter? findChildIndexCallback,
+    int? itemCount,
+    bool addAutomaticKeepAlives = true,
+    bool addRepaintBoundaries = true,
+    bool addSemanticIndexes = true,
+  }) : super(delegate: SliverChildBuilderDelegate(
+    itemBuilder,
+    findChildIndexCallback: findChildIndexCallback,
+    childCount: itemCount,
+    addAutomaticKeepAlives: addAutomaticKeepAlives,
+    addRepaintBoundaries: addRepaintBoundaries,
+    addSemanticIndexes: addSemanticIndexes,
+  ));
+
+  /// A sliver that places multiple box children in a linear array along the main
+  /// axis.
+  ///
+  /// [SliverVariedExtentList] places its children in a linear array along the main
+  /// axis starting at offset zero and without gaps. Each child is forced to have
+  /// the returned extent of [itemExtentBuilder] in the main axis and the
+  /// [SliverConstraints.crossAxisExtent] in the cross axis.
+  ///
+  /// This constructor uses a list of [Widget]s to build the sliver.
+  SliverVariedExtentList.list({
+    super.key,
+    required List<Widget> children,
+    required this.itemExtentBuilder,
+    bool addAutomaticKeepAlives = true,
+    bool addRepaintBoundaries = true,
+    bool addSemanticIndexes = true,
+  }) : super(delegate: SliverChildListDelegate(
+    children,
+    addAutomaticKeepAlives: addAutomaticKeepAlives,
+    addRepaintBoundaries: addRepaintBoundaries,
+    addSemanticIndexes: addSemanticIndexes,
+  ));
+
+  /// The children extent builder.
+  ///
+  /// Should return null if asked to build an item extent with a greater index than
+  /// exists.
+  final ItemExtentBuilder itemExtentBuilder;
+
+  @override
+  RenderSliverVariedExtentList createRenderObject(BuildContext context) {
+    final SliverMultiBoxAdaptorElement element = context as SliverMultiBoxAdaptorElement;
+    return RenderSliverVariedExtentList(childManager: element, itemExtentBuilder: itemExtentBuilder);
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, RenderSliverVariedExtentList renderObject) {
+    renderObject.itemExtentBuilder = itemExtentBuilder;
+  }
+}
+
 /// A sliver that places multiple box children in a two dimensional arrangement.
 ///
 /// _To learn more about slivers, see [CustomScrollView.slivers]._

packages/flutter/lib/widgets.dart

@@ -136,7 +136,6 @@ 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';
-export 'src/widgets/sliver_varied_extent_list.dart';
 export 'src/widgets/slotted_render_object_widget.dart';
 export 'src/widgets/snapshot_widget.dart';
 export 'src/widgets/spacer.dart';