aed327b84 Roll Dart SDK from 4a4c1361ad7b to 4fa07f3ccd43 (1 revision) (flutter/engine#28979) (#91074)

Building an application for a desktop platform that transitively included any Dart-based plugins (such as path_provider) broke `flutter test`, because its compilation was overriding the provided main (in this case, the test main) with `generated_main.dart` if it was present. This PR:
- Changes the `flutter test` compilation path to update `generated_main.dart`, so that the tests will work, and will include any registered Dart plugins.
- Makes using `generated_main.dart` during recompile opt-in, to try to reduce the chance of a similar bug happening with other codepaths in the future.

Fixes https://github.com/flutter/flutter/issues/88794

I've added a Focus.withExternalFocusNode constructor to the Focus widget (and the FocusScope widget) that makes it explicit that the widget's attributes won't affect the settings of the given focus node.

This is to help address #83023, which is a snag in the API that people run into occasionally.

This should help make it explicit when you want the widget attributes to take precedence, and when you don't.

This modifies the flutter update-packages and flutter update-packages --force-upgrade commands so that the many invocations of "dart pub get" in each repo project run in parallel instead of in series.

* Initial layout and theme

* fixed tests

* made page scrollable

* fixes based on Chris on Casey's comments

* removed desktop_window, add errorWidget

* 2nd round of fixes

* fixed linux analyze

* changes based on Casey's comments:

* remove trailing space

* removed obselete macos build code

* add discrepency in project.pbxproj

* addressed Casey's comment

Prevents bugs related to invalid (-1,-1) selection in keyboard shortcuts

* 1b5b3c0b0 Roll Skia from 43264640f256 to 791c0d36a6f6 (2 revisions) (flutter/engine#28909)

* d3f17b01d Mirror master to main branch. (flutter/engine#28896)

* 8f964925e Roll Dart SDK from f452a6585cbd to 280a0c3efb21 (1 revision) (flutter/engine#28910)

* 0aee3c6d1 Roll Skia from 791c0d36a6f6 to 0f124cd7cd60 (2 revisions) (flutter/engine#28911)

* bd250bdd8 Windows: Add dark theme support. (flutter/engine#28131)

This reverts commit ab51a026 and fixes the test that broke the first time it landed.