…utter#100271)

…utter#100271)

…utter#100271)

…utter#100271)

Previously, flutter#100271 enabled
building universal macOS binaries by default, but included a bug causing
the arm64 App.framework to be built such that the TEXT section
containing the app instructions built by gen_snapshot incorrectly
contained x86_64 instructions rather than arm64 instructions.

When building macOS (and iOS) apps, Flutter builds them in three
components:
* The Runner application: built by Xcode
* The bundled App.framework: built from assembly code generated by
  gen_snapshot from the application's Dart sources.
* The bundled FlutterMacOS.framework: built as part of the engine build
  and packaged by copying the distributed binary framework from our
  artifacts cache.

Building App.framework consists of the following steps:
* For each architecture, invoke gen_snapshot to generate
  architecture-specific assembly code, which is then built to object
  code and linked into an architecture-specific App.framework.
* Use the `lipo` tool to generate a universal binary that includes both
  x86_64 and arm64 architectures.

Previously, we were building architecture specific App.framework
binaries. However, for all architectures we were (mistakenly) invoking
the general `gen_snapshot` tool (which emitted x64 instructions, and
which is now deprecated) instead of the architecture-specific
`gen_snapshot_x86` and `gen_snapshot_arm64` builds which emit
instructions for the correct architecture.

This change introduces a small refactoring, which is to split the
`getNameForDarwinArch` function into two functions:
* `getDartNameForDarwinArch`: the name for the specified architecture as
  used in the Dart SDK, for example as the suffix of `gen_snapshot`.
* `getNameForDarwinArch`: the name for the specified architecture
  as used in Apple tools, for example as an argument to `lipo`. For
  consistency, and to match developer expectations on Darwin platforms,
  this is also the name used in Flutter's build outputs.

Issue: flutter#100348

Previously, #100271 enabled
building universal macOS binaries by default, but included a bug causing
the arm64 App.framework to be built such that the TEXT section
containing the app instructions built by gen_snapshot incorrectly
contained x86_64 instructions rather than arm64 instructions.

When building macOS (and iOS) apps, Flutter builds them in three
components:
* The Runner application: built by Xcode
* The bundled App.framework: built from assembly code generated by
  gen_snapshot from the application's Dart sources.
* The bundled FlutterMacOS.framework: built as part of the engine build
  and packaged by copying the distributed binary framework from our
  artifacts cache.

Building App.framework consists of the following steps:
* For each architecture, invoke gen_snapshot to generate
  architecture-specific assembly code, which is then built to object
  code and linked into an architecture-specific App.framework.
* Use the `lipo` tool to generate a universal binary that includes both
  x86_64 and arm64 architectures.

Previously, we were building architecture specific App.framework
binaries. However, for all architectures we were (mistakenly) invoking
the general `gen_snapshot` tool (which emitted x64 instructions, and
which is now deprecated) instead of the architecture-specific
`gen_snapshot_x86` and `gen_snapshot_arm64` builds which emit
instructions for the correct architecture.

This change introduces a small refactoring, which is to split the
`getNameForDarwinArch` function into two functions:
* `getDartNameForDarwinArch`: the name for the specified architecture as
  used in the Dart SDK, for example as the suffix of `gen_snapshot`.
* `getNameForDarwinArch`: the name for the specified architecture
  as used in Apple tools, for example as an argument to `lipo`. For
  consistency, and to match developer expectations on Darwin platforms,
  this is also the name used in Flutter's build outputs.

Issue: #100348

Make the `NativeAssets` target consistent with `build_info.dart`'s
documentation on missing `IosArchs` or `DarwinArchs`.

Bug:

* #136931

Also, updates the doc to reflect that MacOS is by default built for both
x64 and arm64. The PR making universal binaries the default didn't
update the doc comment.

* #100271

Please note that these defines are handled inconsistently in
`flutter_tools`. In some places they default to what's specified in the
doc-comment. In other places a `MissingDefineException` is thrown. I
believe the code around `build_info.dart` and the `environment` could
benefit from a wrapping so that defaults or missing definitions are
handled consistently in the code base.

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- [x] I read the [Contributor Guide] and followed the process outlined
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- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] All existing and new tests are passing.

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