This fixes an issue with the Dart Head-Head-Head CI bot, where it was seeing failures because of concurrent execution of the snippets tool.

It adds a single run of the snippets tool right after activation to force the compiling of the package executable, which is what has trouble with concurrent invocation.

This is a temporary measure until dart pub global run is fixed.

This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.

I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.

I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.

6f4143e80 Roll Dart SDK from cea2ea76fc66 to d50eacfa4fcd (2 revisions) (flutter/engine#28988) (#91123)

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.