feat: Expose FocusNode of FocusTraversalGroup (#171979)

Expose the focus node of `FocusTraversalGroup` so customers can listen
to focus changes of its descendants.

Fixes https://github.com/flutter/flutter/issues/171516

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

@@ -2038,6 +2038,7 @@ class FocusTraversalGroup extends StatefulWidget {
     FocusTraversalPolicy? policy,
     this.descendantsAreFocusable = true,
     this.descendantsAreTraversable = true,
+    this.onFocusNodeCreated,
     required this.child,
   }) : policy = policy ?? ReadingOrderTraversalPolicy();
 
@@ -2069,6 +2070,9 @@ class FocusTraversalGroup extends StatefulWidget {
   /// {@macro flutter.widgets.ProxyWidget.child}
   final Widget child;
 
+  /// Called when the [FocusNode] of this widget is created.
+  final void Function(FocusNode)? onFocusNodeCreated;
+
   /// Returns the [FocusTraversalPolicy] that applies to the nearest ancestor of
   /// the given [FocusNode].
   ///
@@ -2206,6 +2210,12 @@ class _FocusTraversalGroupState extends State<FocusTraversalGroup> {
     policy: widget.policy,
   );
 
+  @override
+  void initState() {
+    super.initState();
+    widget.onFocusNodeCreated?.call(focusNode);
+  }
+
   @override
   void dispose() {
     focusNode.dispose();

packages/flutter/test/widgets/focus_traversal_test.dart

@@ -3786,6 +3786,78 @@ void main() {
     expect(childScope.hasFocus, isTrue);
     expect(nodeA.hasFocus, isFalse);
   });
+
+  testWidgets('GIVEN onFocusNodeCreated is not null '
+      'THEN it is called when the FocusTraversalGroup is built', (WidgetTester tester) async {
+    FocusNode? node;
+    await tester.pumpWidget(
+      FocusTraversalGroup(
+        onFocusNodeCreated: (FocusNode createdNode) => node = createdNode,
+        child: const SizedBox.shrink(),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(node, isNotNull);
+  });
+
+  testWidgets(
+    'GIVEN a FocusScope with no focusable descendants '
+    'WHEN the user presses TAB to navigate focus '
+    'THEN focus should skip the scope and land on the next focusable widget without requiring multiple TAB presses',
+    (WidgetTester tester) async {
+      final FocusNode enabledButton1Node = FocusNode();
+      addTearDown(enabledButton1Node.dispose);
+
+      final FocusNode enabledButton2Node = FocusNode();
+      addTearDown(enabledButton2Node.dispose);
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Center(
+            child: Column(
+              mainAxisSize: MainAxisSize.min,
+              children: <Widget>[
+                MaterialButton(
+                  focusNode: enabledButton1Node,
+                  onPressed: () {}, // enabled
+                  child: const Text('Enabled Button 1'),
+                ),
+                FocusTraversalGroup(
+                  child: const Column(
+                    mainAxisSize: MainAxisSize.min,
+                    children: <Widget>[
+                      MaterialButton(
+                        onPressed: null, // disabled
+                        child: Text('Disabled Button 1'),
+                      ),
+                      SizedBox(height: 16),
+                      MaterialButton(
+                        onPressed: null, // disabled
+                        child: Text('Disabled Button 2'),
+                      ),
+                    ],
+                  ),
+                ),
+                MaterialButton(
+                  focusNode: enabledButton2Node,
+                  onPressed: () {}, // enabled
+                  child: const Text('Enabled Button 2'),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.pumpAndSettle();
+      expect(enabledButton1Node.hasPrimaryFocus, isTrue);
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
+      await tester.pumpAndSettle();
+      expect(enabledButton2Node.hasPrimaryFocus, isTrue);
+    },
+  );
 }
 
 class TestRoute extends PageRouteBuilder<void> {