Support for FFI calls with `@Native external` functions through Native assets on MacOS and iOS. This enables bundling native code without any build-system boilerplate code.

For more info see:

* https://github.com/flutter/flutter/issues/129757

### Implementation details for MacOS and iOS.

Dylibs are bundled by (1) making them fat binaries if multiple architectures are targeted, (2) code signing these, and (3) copying them to the frameworks folder. These steps are done manual rather than via CocoaPods. CocoaPods would have done the same steps, but (a) needs the dylibs to be there before the `xcodebuild` invocation (we could trick it, by having a minimal dylib in the place and replace it during the build process, that works), and (b) can't deal with having no dylibs to be bundled (we'd have to bundle a dummy dylib or include some dummy C code in the build file).

The dylibs are build as a new target inside flutter assemble, as that is the moment we know what build-mode and architecture to target.

The mapping from asset id to dylib-path is passed in to every kernel compilation path. The interesting case is hot-restart where the initial kernel file is compiled by the "inner" flutter assemble, while after hot restart the "outer" flutter run compiled kernel file is pushed to the device. Both kernel files need to contain the mapping. The "inner" flutter assemble gets its mapping from the NativeAssets target which builds the native assets. The "outer" flutter run get its mapping from a dry-run invocation. Since this hot restart can be used for multiple target devices (`flutter run -d all`) it contains the mapping for all known targets.

### Example vs template

The PR includes a new template that uses the new native assets in a package and has an app importing that. Separate discussion in: https://github.com/flutter/flutter/issues/131209.

### Tests

This PR adds new tests to cover the various use cases.

* dev/devicelab/bin/tasks/native_assets_ios.dart
  * Runs an example app with native assets in all build modes, doing hot reload and hot restart in debug mode.
* dev/devicelab/bin/tasks/native_assets_ios_simulator.dart
  * Runs an example app with native assets, doing hot reload and hot restart.
* packages/flutter_tools/test/integration.shard/native_assets_test.dart
  * Runs (incl hot reload/hot restart), builds, builds frameworks for iOS, MacOS and flutter-tester.
* packages/flutter_tools/test/general.shard/build_system/targets/native_assets_test.dart
  * Unit tests the new Target in the backend.
* packages/flutter_tools/test/general.shard/ios/native_assets_test.dart
* packages/flutter_tools/test/general.shard/macos/native_assets_test.dart
  * Unit tests the native assets being packaged on a iOS/MacOS build.

It also extends various existing tests:

* dev/devicelab/bin/tasks/module_test_ios.dart
   * Exercises the add2app scenario.
* packages/flutter_tools/test/general.shard/features_test.dart
   * Unit test the new feature flag.

PLUS: clean-up of all the unreachable stuff.

flutter_tools: Flutter Web CLI cleanup

Revert "[flutter_tools] Remove sound null safety flag (#123031)"

[flutter_tools] Remove sound null safety flag #2

This reverts commit 7c3088cf.

[flutter_tools] Remove sound null safety flag

* [web] Put all index.html operations in one place

* review comments

* fix build

* change quotes

* fix test

* Remove superfluous words.

* Update packages/flutter/lib/src/widgets/slotted_render_object_widget.dart
Co-authored-by: Michael Goderbauer <goderbauer@google.com>

Update test expectations from application/javascript -> text/javascript

`package:mime` now uses `text/javascript` and not `application/javascript`.

See https://github.com/dart-lang/mime/pull/76.
See https://datatracker.ietf.org/doc/html/rfc9239.

> This document defines equivalent processing requirements for the various script media types. The most widely supported media type in use is `text/javascript`; all others are considered historical and obsolete aliases of `text/javascript`.

* Migrate packages/flutter_tools/test/general.shard, part 2/2

* Fix analysis

* Fix tests

* Fix analysis

* Apply suggestions from code review
Co-authored-by: Christopher Fujino <fujino@google.com>
Co-authored-by: Christopher Fujino <fujino@google.com>

Revert "Read dart_plugin_registrant path from FlutterProject to support non-standard path." (#107850)

* Provide flutter sdk kernel files to dwds launcher instead of dart ones

* Update log test to report all warnings

* Update licences for new files

* Addressed CR comments

* Addressed CR comments

This reverts commit 5fd259be.

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