TreeSliver & associated classes (#147171)

**FYI for Reviewers:** Much of the API surface matches that of the 2D TreeView in flutter#6592. If it changes here, it should change there, and vice versa.

�  [Design Document](https://docs.google.com/document/d/1-aFI7VjkF9yMkWpP94J8T_JREDS-M3bOak26PVehUYg/edit?usp=sharing)

This adds classes and associated callbacks and controllers for TreeSliver. Core components:
- TreeSliver
- RenderTreeSliver
- TreeSliverNode
- TreeSliverController
- TreeSliverStateMixin
- TreeSliverIndentationType

Fixes flutter/flutter#114299

https://github.com/flutter/flutter/assets/16964204/3facd095-7262-4068-aa33-d713e2deca99

https://github.com/flutter/flutter/assets/16964204/f851ae30-8e71-45c7-82a4-9606986a5872

examples/api/lib/widgets/sliver/sliver_tree.0.dart

@@ -0,0 +1,104 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+/// Flutter code sample for [TreeSliver].
+
+void main() => runApp(const TreeSliverExampleApp());
+
+class TreeSliverExampleApp extends StatelessWidget {
+  const TreeSliverExampleApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: TreeSliverExample(),
+    );
+  }
+}
+
+class TreeSliverExample extends StatefulWidget {
+  const TreeSliverExample({super.key});
+
+  @override
+  State<TreeSliverExample> createState() => _TreeSliverExampleState();
+}
+
+class _TreeSliverExampleState extends State<TreeSliverExample> {
+  TreeSliverNode<String>? _selectedNode;
+  final TreeSliverController controller = TreeSliverController();
+  final List<TreeSliverNode<String>> _tree = <TreeSliverNode<String>>[
+    TreeSliverNode<String>('First'),
+    TreeSliverNode<String>(
+      'Second',
+      children: <TreeSliverNode<String>>[
+        TreeSliverNode<String>(
+          'alpha',
+          children: <TreeSliverNode<String>>[
+            TreeSliverNode<String>('uno'),
+            TreeSliverNode<String>('dos'),
+            TreeSliverNode<String>('tres'),
+          ],
+        ),
+        TreeSliverNode<String>('beta'),
+        TreeSliverNode<String>('kappa'),
+      ],
+    ),
+    TreeSliverNode<String>(
+      'Third',
+      expanded: true,
+      children: <TreeSliverNode<String>>[
+        TreeSliverNode<String>('gamma'),
+        TreeSliverNode<String>('delta'),
+        TreeSliverNode<String>('epsilon'),
+      ],
+    ),
+    TreeSliverNode<String>('Fourth'),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text('TreeSliver Demo'),
+      ),
+      body: CustomScrollView(
+        slivers: <Widget>[
+          TreeSliver<String>(
+            tree: _tree,
+            controller: controller,
+            treeNodeBuilder: (
+              BuildContext context,
+              TreeSliverNode<Object?> node,
+              AnimationStyle animationStyle,
+            ) {
+              Widget child = GestureDetector(
+                behavior: HitTestBehavior.translucent,
+                onTap: () {
+                  setState(() {
+                    controller.toggleNode(node);
+                    _selectedNode = node as TreeSliverNode<String>;
+                  });
+                },
+                child: TreeSliver.defaultTreeNodeBuilder(
+                  context,
+                  node,
+                  animationStyle,
+                ),
+              );
+              if (_selectedNode == node as TreeSliverNode<String>) {
+                child = ColoredBox(
+                  color: Colors.purple[100]!,
+                  child: child,
+                );
+              }
+              return child;
+            },
+          ),
+        ],
+      ),
+    );
+  }
+}

examples/api/lib/widgets/sliver/sliver_tree.1.dart

@@ -0,0 +1,189 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+
+/// Flutter code sample for [TreeSliver].
+
+void main() => runApp(const TreeSliverExampleApp());
+
+class TreeSliverExampleApp extends StatelessWidget {
+  const TreeSliverExampleApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: TreeSliverExample(),
+    );
+  }
+}
+
+class TreeSliverExample extends StatefulWidget {
+  const TreeSliverExample({super.key});
+
+  @override
+  State<TreeSliverExample> createState() => _TreeSliverExampleState();
+}
+
+class _TreeSliverExampleState extends State<TreeSliverExample> {
+  TreeSliverNode<String>? _selectedNode;
+  final List<TreeSliverNode<String>> tree = <TreeSliverNode<String>>[
+    TreeSliverNode<String>('README.md'),
+    TreeSliverNode<String>('analysis_options.yaml'),
+    TreeSliverNode<String>(
+      'lib',
+      children: <TreeSliverNode<String>>[
+        TreeSliverNode<String>(
+          'src',
+          children: <TreeSliverNode<String>>[
+            TreeSliverNode<String>(
+              'widgets',
+              children: <TreeSliverNode<String>>[
+                TreeSliverNode<String>('about.dart.dart'),
+                TreeSliverNode<String>('app.dart'),
+                TreeSliverNode<String>('basic.dart'),
+                TreeSliverNode<String>('constants.dart'),
+              ],
+            ),
+          ],
+        ),
+        TreeSliverNode<String>('widgets.dart'),
+      ],
+    ),
+    TreeSliverNode<String>('pubspec.lock'),
+    TreeSliverNode<String>('pubspec.yaml'),
+    TreeSliverNode<String>(
+      'test',
+      children: <TreeSliverNode<String>>[
+        TreeSliverNode<String>(
+          'widgets',
+          children: <TreeSliverNode<String>>[
+            TreeSliverNode<String>('about_test.dart'),
+            TreeSliverNode<String>('app_test.dart'),
+            TreeSliverNode<String>('basic_test.dart'),
+            TreeSliverNode<String>('constants_test.dart'),
+          ],
+        ),
+      ],
+    ),
+  ];
+
+  Widget _treeNodeBuilder(
+    BuildContext context,
+    TreeSliverNode<Object?> node,
+    AnimationStyle toggleAnimationStyle,
+  ) {
+    final bool isParentNode = node.children.isNotEmpty;
+    final BorderSide border = BorderSide(
+      width: 2,
+      color: Colors.purple[300]!,
+    );
+    return TreeSliver.wrapChildToToggleNode(
+      node: node,
+      child: Row(
+        children: <Widget>[
+          // Custom indentation
+          SizedBox(width: 10.0 * node.depth! + 8.0),
+          DecoratedBox(
+            decoration: BoxDecoration(
+              border: node.parent != null
+                  ? Border(left: border, bottom: border)
+                  : null,
+            ),
+            child: const SizedBox(height: 50.0, width: 20.0),
+          ),
+          // Leading icon for parent nodes
+          if (isParentNode)
+            DecoratedBox(
+              decoration: BoxDecoration(border: Border.all()),
+              child: SizedBox.square(
+                dimension: 20.0,
+                child: Icon(
+                  node.isExpanded ? Icons.remove : Icons.add,
+                  size: 14,
+                ),
+              ),
+            ),
+          // Spacer
+          const SizedBox(width: 8.0),
+          // Content
+          Text(node.content.toString()),
+        ],
+      ),
+    );
+  }
+
+  Widget _getTree() {
+    return DecoratedSliver(
+      decoration: BoxDecoration( border: Border.all()),
+      sliver: TreeSliver<String>(
+        tree: tree,
+        onNodeToggle: (TreeSliverNode<Object?> node) {
+          setState(() {
+            _selectedNode = node as TreeSliverNode<String>;
+          });
+        },
+        treeNodeBuilder: _treeNodeBuilder,
+        treeRowExtentBuilder: (
+          TreeSliverNode<Object?> node,
+          SliverLayoutDimensions layoutDimensions,
+        ) {
+          // This gives more space to parent nodes.
+          return node.children.isNotEmpty ? 60.0 : 50.0;
+        },
+        // No internal indentation, the custom treeNodeBuilder applies its
+        // own indentation to decorate in the indented space.
+        indentation: TreeSliverIndentationType.none,
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    // This example is assumes the full screen is available.
+    final Size screenSize = MediaQuery.sizeOf(context);
+    final List<Widget> selectedChildren = <Widget>[];
+    if (_selectedNode != null) {
+      selectedChildren.addAll(<Widget>[
+        const Spacer(),
+        Icon(
+          _selectedNode!.children.isEmpty
+              ? Icons.file_open_outlined
+              : Icons.folder_outlined,
+        ),
+        const SizedBox(height: 16.0),
+        Text(_selectedNode!.content),
+        const Spacer(),
+      ]);
+    }
+    return Scaffold(
+      body: Row(children: <Widget>[
+        SizedBox(
+          width: screenSize.width / 2,
+          height: double.infinity,
+          child: CustomScrollView(
+            slivers: <Widget>[
+              _getTree(),
+            ],
+          ),
+        ),
+        DecoratedBox(
+          decoration: BoxDecoration(
+            border: Border.all(),
+          ),
+          child: SizedBox(
+            width: screenSize.width / 2,
+            height: double.infinity,
+            child: Center(
+              child: Column(
+                children: selectedChildren,
+              ),
+            ),
+          ),
+        ),
+      ]),
+    );
+  }
+}

examples/api/test/widgets/sliver/sliver_tree.0_test.dart

@@ -0,0 +1,21 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_api_samples/widgets/sliver/sliver_tree.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Can toggle nodes in TreeSliver', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TreeSliverExampleApp(),
+    );
+    expect(find.text('Second'), findsOneWidget);
+    expect(find.text('alpha'), findsNothing);
+    // Toggle tree node.
+    await tester.tap(find.text('Second'));
+    await tester.pumpAndSettle();
+    expect(find.text('alpha'), findsOneWidget);
+  });
+}

examples/api/test/widgets/sliver/sliver_tree.1_test.dart

@@ -0,0 +1,21 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_api_samples/widgets/sliver/sliver_tree.1.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Can toggle nodes in TreeSliver', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TreeSliverExampleApp(),
+    );
+    expect(find.text('lib'), findsOneWidget);
+    expect(find.text('src'), findsNothing);
+    // Toggle tree node.
+    await tester.tap(find.text('lib'));
+    await tester.pumpAndSettle();
+    expect(find.text('src'), findsOneWidget);
+  });
+}

packages/flutter/lib/rendering.dart

@@ -67,6 +67,7 @@ export 'src/rendering/sliver_list.dart';
 export 'src/rendering/sliver_multi_box_adaptor.dart';
 export 'src/rendering/sliver_padding.dart';
 export 'src/rendering/sliver_persistent_header.dart';
+export 'src/rendering/sliver_tree.dart';
 export 'src/rendering/stack.dart';
 export 'src/rendering/table.dart';
 export 'src/rendering/table_border.dart';

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

@@ -8,6 +8,7 @@ import 'package:vector_math/vector_math_64.dart';
 import 'box.dart';
 import 'object.dart';
 import 'sliver.dart';
+import 'sliver_fixed_extent_list.dart';
 
 /// A delegate used by [RenderSliverMultiBoxAdaptor] to manage its children.
 ///