This re-lands #49235 with the addition of includeSemantics flag on the Focus widget so that the FocusTraversalGroup can create a Focus widget without affecting the semantics tree.

The FocusTraversalGroup uses the Focus widget to create a grouping of descendants for traversal, but doesn't actually participate in focus (canRequestFocus is always false), so we don't want it to add a Semantics widget in that case, since that can cause semantics changes. The canRequestFocus attribute can also be used when a widget is disabled, so we do sometimes want to include Semantics even if that is false, but not in the case where it is always false, as for FocusTraversalGroup.

- Added a test to make sure that FocusTraversalGroup doesn't add any semantics information.

This fixes a problem when unfocusing focus nodes where the ancestor focus nodes and scopes don't receive notification that a child was unfocused.

Fixes #43497

* f49a8b62f Roll src/third_party/skia c03e6982f96f..465864cad5d2 (14 commits) (flutter/engine#16524)

* c477c0644 Enable verbose logging for shell unittests on Fuchsia (flutter/engine#16526)

* a662579f7 Clear frame references at the end of every CanvasKit frame (flutter/engine#16525)

* 3f31ea3d3 Roll src/third_party/skia 465864cad5d2..21f382c19d76 (6 commits) (flutter/engine#16528)

* 38fb6b135 Roll fuchsia/sdk/core/linux-amd64 from 8L7NY... to Bmq1m... (flutter/engine#16529)

* 9c0168a5a Roll fuchsia/sdk/core/mac-amd64 from PMcw3... to 7JkB7... (flutter/engine#16530)

* e8a888dfa Roll src/third_party/skia 21f382c19d76..f83d0346c06a (2 commits) (flutter/engine#16532)

* 1e8b33130 Roll src/third_party/dart 5244d99a5d4e..5fc031ebc1d7 (42 commits) (flutter/engine#16533)

* c4e3ae6ae Roll src/third_party/skia f83d0346c06a..88c3793a4eaa (1 commits) (flutter/engine#16534)

* 6cdb14e17 Roll src/third_party/skia 88c3793a4eaa..abefc9c170c9 (1 commits) (flutter/engine#16535)

* 975acd8ff Roll src/third_party/skia abefc9c170c9..4fe89b4d871d (2 commits) (flutter/engine#16536)

* b7424d0ce Roll src/third_party/dart 5fc031ebc1d7..30151a654151 (2 commits) (flutter/engine#16537)

* 25e812778 Roll src/third_party/skia 4fe89b4d871d..dc2782c380f6 (1 commits) (flutter/engine#16538)

* 74fa10c95 Roll src/third_party/dart 30151a654151..76b18c455e2c (1 commits) (flutter/engine#16539)

* 91b8e406c Roll src/third_party/skia dc2782c380f6..cdf2491afa04 (1 commits) (flutter/engine#16540)

* 5acf9b195 Roll src/third_party/skia cdf2491afa04..50a490a1a4fb (2 commits) (flutter/engine#16541)

* 989777718 Roll src/third_party/skia 50a490a1a4fb..c3b67eb988c8 (4 commits) (flutter/engine#16542)

* 78a890972 Use os_log instead of syslog on Apple platforms (flutter/engine#13487)

* ea56ad22d libtxt: use a fixture in the benchmarks (flutter/engine#16531)

* a61dbf2f6 Revert "Use os_log instead of syslog on Apple platforms (#13487)" (flutter/engine#16546)

* 539f64f56 [fuchsia] Disable retained layers (flutter/engine#16548)

* c3b507248 Expose DPI helper functions for Runner apps to use (flutter/engine#16313)

* 5041ff1b1 support endless trace buffer (flutter/engine#16520)

* 6aacf5e76 Re-land: Use os_log instead of syslog on Apple platforms (flutter/engine#16549)

* a5736b85d Roll src/third_party/skia c3b67eb988c8..b1525c721ea6 (4 commits) (flutter/engine#16543)

* 49a370f8e Roll src/third_party/dart 76b18c455e2c..e4c39721c473 (6 commits) (flutter/engine#16544)

* 270421c10 Fix ensureInitializationCompleteAsync callback when already initialized. (#39675) (flutter/engine#16503)

* ca02b9197 Prevent long flash when switching to Flutter app. (#47903) (flutter/engine#16527)

* 44e80fd61 skiping tests in Safari. LUCI recipe for Mac is ready. this is the only step left for stopping us running unit tests in Safari (flutter/engine#16550)

* 5fb0116d6 iOS platform view gesture blocking policy. (flutter/engine#15940)

* e0ebaea59 Revert "Re-land: Use os_log instead of syslog on Apple platforms (#16549)" (flutter/engine#16558)

Revert "AlertDialog.actionsOverflowButtonSpacing and ButtonBar.overflowButtonSpacing (#50609)" (#50674)

* Add ButtonBar.overflowButtonSpacing

* Add AlertDialog overflow button spacing functionality

This reverts commit 8ef5e2f0 because it breaks some semantics tests.

* a195a92cc [SkParagraph] Text layout benchmarks using SkParagraph (flutter/engine#16373)

* 0340d457a Roll fuchsia/sdk/core/mac-amd64 from fvWgE... to FLFOw... (flutter/engine#16498)

* 9443518f6 Fix safe area reporting with transparent FlutterActivitys. (#46060) (flutter/engine#16208)

* 0980b2ad5 Roll src/third_party/skia 9f3eef796f63..b2031d4255e2 (12 commits) (flutter/engine#16499)

* 8f03a563d Check Java formatting in format.sh (flutter/engine#16500)

* 077918dcf Apply Google Java format (flutter/engine#16501)

* ec1b2e39f Roll src/third_party/skia b2031d4255e2..a9405c2a2967 (5 commits) (flutter/engine#16502)

* 82d193241 Roll src/third_party/skia a9405c2a2967..8dad1ffd81f8 (1 commits) (flutter/engine#16505)

* a302bfd06 Roll src/third_party/dart fd992e423ef6..4310f50edc95 (17 commits) (flutter/engine#16506)

* a86f4cba3 Roll fuchsia/sdk/core/linux-amd64 from LvSlH... to 8L7NY... (flutter/engine#16508)

* 6c04a6960 Roll src/third_party/skia 8dad1ffd81f8..c03e6982f96f (2 commits) (flutter/engine#16510)

* b1aaec3c4 Roll src/third_party/dart 4310f50edc95..9464eb51f539 (2 commits) (flutter/engine#16511)

* cab49e420 Roll fuchsia/sdk/core/mac-amd64 from FLFOw... to PMcw3... (flutter/engine#16512)

* 695a2768e Roll src/third_party/dart 9464eb51f539..f120417ddbfc (1 commits) (flutter/engine#16513)

* 6d3e2606e Roll src/third_party/dart f120417ddbfc..b158bf7bd537 (2 commits) (flutter/engine#16514)

* acc26a2f1 Use Skia to determine "actual" font name in CanvasKit backend (flutter/engine#16504)

* 1daba5318 Partially fix canvas vs text paint order when running on Blink/Webkit browsers (flutter/engine#16483)

* 826b2d46e Apply lints from internal roll (flutter/engine#16509)

* d107eaaa0 Remove mirrors on the web (flutter/engine#16516)

* 7e869faf5 Adds new route annoucement method. (flutter/engine#16523)

* 0e8ed2715 Roll src/third_party/dart b158bf7bd537..5244d99a5d4e (1 commits) (flutter/engine#16517)

This reverts commit 0a38f29c.

Revert "Revert "Reduced code duplication in ink_splash.dart, ink_ripple.dart (#49480)" (#50502)" (#50512)

This reverts commit 9bc0e6a9.

Updates the README file for snippets.

This change adds a way to provide explicit focus order for a part of the widget tree.

It adds FocusTraversalPolicyGroup, which in many ways is similar to DefaultFocusTraversal, except that it groups a widget subtree together so that those nodes are traversed as a group. DefaultFocusTraversal doesn't work as one would expect: If there is more than one DefaultFocusTraversal inside of a focus scope, the policy can change depending on which node was asked to move "next", which can cause unexpected behavior. The new grouping mechanism doesn't have that problem. I deprecate DefaultFocusTraversal in this PR.

It also adds OrderedFocusTraversalPolicy, which is a policy that can be supplied to FocusTraversalPolicyGroup to set the policy for a sub-tree. It looks for FocusTraversalOrder inherited widgets, which use a FocusOrder to do the sorting. FocusOrder has two subclasses: NumericalFocusOrder (which sorts based on a double), and LexicalFocusOrder, which sorts based on a String.

As part of doing this, I refactored the way FocusTraversalPolicy is implemented so that it has more default implementation methods, and exposes a new protected member: sortDescendants, which makes it easier for developers to make their own policy subclasses: they only need to implement sortDescendants to get a new ordering behavior, but can also still override any of the default implementation behaviors if they need different behavior.

I was able to do this without breaking the API (AFAICT).