Towards #184077

I need to try another solution first though.

Summary
This PR improves the accessibility of the Scaffold widget by explicitly defining the focus and
semantics traversal order for its various slots. This ensures that users of assistive technologies
(keyboard navigation and screen readers) encounter elements in a logical, top-down order,
prioritizing navigation and primary actions before main content.

Related Issue
Fixes #184077

The Problem
Previously, the traversal order in a Scaffold was determined by the build/layout order within its
internal CustomMultiChildLayout. This often resulted in the body being traversed before the
FloatingActionButton (FAB) or BottomNavigationBar. If the body contained a long scrollable list,
users were effectively "trapped" in the content and forced to navigate through every list item
before they could reach global navigation or the primary page action (FAB).

The Solution: "Chrome-before-Content"
This change implements a "Chrome-before-Content" traversal pattern. By prioritizing the AppBar,
BottomNavigationBar, and FloatingActionButton before the body, users can quickly switch context or
perform primary actions without traversing the entire main content area.

The final defined order is:

1. MaterialBanner (1.0): Topmost urgent notifications.
2. AppBar (2.0): Page context and top navigation.
3. BottomNavigationBar (3.0): Global app-level navigation.
4. FloatingActionButton (4.0): Primary page-level action.
5. Body (5.0): Main content area.
6. BottomSheet (6.0): Supplemental content/actions.
7. SnackBar (7.0): Transient status feedback.
8. PersistentFooterButtons (8.0): Secondary actions following the body (e.g., a "Submit" button).

Key Navigation Benefits:

• Forward Navigation (Tab): Users reach global navigation and the primary FAB immediately after
  the AppBar, before entering the body.
• Backward Navigation (Shift+Tab): Users can Shift-Tab from the start of the body to quickly jump
  back to the primary action or navigation.

Technical Implementation

• Scaffold: Wrapped the layout area in a FocusTraversalGroup with OrderedTraversalPolicy and a
  Semantics container to enable sorting.
• _addIfNonNull: Updated this internal helper to automatically apply FocusTraversalOrder
  (NumericFocusOrder) and Semantics.sortKey (OrdinalSortKey) to each slot.
• SemanticsTester: Added support for tooltip matching in the includesNodeWith matcher, enabling
  cleaner and more robust semantics tests for widgets like IconButton.

Tests
New Tests

• packages/flutter/test/material/scaffold_navigation_order_test.dart: Explicitly verifies the
  full forward/backward Tab sequence and the correct assignment of semantics sort keys.

Updated Tests
Several existing regression tests were updated to accommodate the new semantics tree structure
(which now includes sorting nodes):

• selectable_region_test.dart: Updated to use Material instead of Scaffold to preserve its
  original intent of testing unmerged semantics nodes without interference from Scaffold's
  internal containers.
• search_test.dart: Refactored to use the improved includesNodeWith matcher, making the tests
  more robust and focused on behavior rather than exact tree depth.
• floating_action_button_test.dart, bottom_navigation_bar_test.dart,
  flexible_space_bar_test.dart, dropdown_menu_test.dart, and navigation_rail_test.dart: Updated
  to match the new semantics nesting while preserving their original verification logic.

Verified that all updated tests and related suites (app_bar_test.dart, banner_test.dart,
snack_bar_test.dart, scaffold_test.dart) pass.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• 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.
• All existing and new tests are passing.

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