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.

Addresses the other part of https://github.com/flutter/flutter/issues/132711

This PR adds uses of the `--target-os` command line argument when
building kernel sources for precompiled applications for supported
target operating systems. The Dart CFE then:

* treats `Platform.operatingSystem` as if it were defined as the
constant string provided as an argument to the flag,
* treats `Platform.pathSeparator` as the appropriate separator for that
operating system,
* attempts to constant evaluate the initializer for any field annotated
with the `vm:platform-const` pragma, and
* attempts to constant evaluate all calls to a method annotated with the
`vm:platform-const` pragma.

The `vm:platform-const` pragma can appear in either library or user
code. If the attempt to constant evaluate the field initializer or
method call fails, then an error is thrown at kernel compilation time.

Addresses #14233.

The tests in
`packages/flutter_tools/test/general.shard/build_system/targets/common_test.dart`
have been adjusted properly to account for the new passed command line
arguments.

Always pass the space code point (`0x20`) to the font subsetter when targetting web because the web engine relies on that character to calculate a font's height.

Fixes #127270

The newly updated lint will soon flag for-each in collections.

See discussion: https://github.com/dart-lang/linter/pull/4383

/cc @goderbauer

Fixes https://github.com/flutter/flutter/issues/126705

Follow up fix after https://github.com/flutter/flutter/pull/126875 did NOT work.

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

All invocations are logged on verbose
Better, more detailed exceptions are thrown on errors
A lot less boilerplate

Allows controlling a broader set of variables than just on/off.

Also make wasm-opt "full" the default

Also update the order of args to commands so that testing has less repeated "stuff"

Refactor "gen-l10n" command to use same code path when "l10n.yaml" is present or not present (#125429)

I think this is a long needed change to the `gen-l10n` command.
Essentially, the arguments to `flutter gen-l10n` can be provided by two
different methods: via command line arguments or via the `l10n.yaml`
file. The existence of a `l10n.yaml` file causes the latter approach to
take precedence.

However, currently, there's several differences in how the two
approaches are handled, and most of the default arguments are all over
the place, causing unexpected issues such as #120457 or #120023.

This PR refactors the command so that
* `LocalizationOptions` are more consistent with the actual argument
names/yaml options.
* All default values are determined in `LocalizationOptions`'s
constructor (or in `argParser.addOption(...)` in the case a boolean
value needs to be explicitly true).
* New `parseLocalizationsOptionsFromCommand` function to parse
arguments.
* Parse `LocalizationOptions` at the beginning of `runCommand()` and
pass it to `generateLocalizations`.

Fixes #120023.

This fixes https://github.com/flutter/flutter/issues/124159

Adds the command line argument `--wasm-opt` to optimize the web assembly output.

Remove Finder extended attributes in build target before code signing iOS frameworks

flutter-tool: enum cleanup

Add omit-type-checks flag for wasm builds

Add support to the flutter tool to compile against the skwasm renderer

flutter_tool: add and use WebCompilerConfig

Always use the canvaskit path from the web sdk.

Invalidate the `WebStaticAssets` target if the web sdk changes.

Add flags to make App.framework work in app extension

Remove 1745 decorative breaks

Use depfile with dart2wasm to determine when to rebuild.

[tool][web] Pass invoker flag to dart2js.

Revert "[web:tools] always use CanvasKit from the cache when building web apps (#93002)"

[flutter_tool] advertise the default value for --dart2js-optimization

* lerp documentation

* Remove Note, Note That from repo

* Improve BorderSide documentation.

* apply review comments

This reverts commit 0fb4406c.

This reverts commit e03029ef.

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

* review comments

* fix build

* change quotes

* fix test

* dart fix --apply

* manual fixes

* fix after merge conflicts

* review