[macOS] Use arm64 snapshot in arm64 App.framework

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

packages/flutter_tools/lib/src/base/build.dart

@@ -64,10 +64,12 @@ class GenSnapshot {
 
     String snapshotterPath = getSnapshotterPath(snapshotType);
 
-    // iOS has a separate gen_snapshot for armv7 and arm64 in the same,
-    // directory. So we need to select the right one.
-    if (snapshotType.platform == TargetPlatform.ios) {
-      snapshotterPath += '_${getNameForDarwinArch(darwinArch!)}';
+    // iOS and macOS have separate gen_snapshot binaries for each target
+    // architecture (iOS: armv7, arm64; macOS: x86_64, arm64). Select the right
+    // one for the target architecture in question.
+    if (snapshotType.platform == TargetPlatform.ios ||
+        snapshotType.platform == TargetPlatform.darwin) {
+      snapshotterPath += '_${getDartNameForDarwinArch(darwinArch!)}';
     }
 
     return _processUtils.stream(

packages/flutter_tools/lib/src/build_info.dart

@@ -570,6 +570,31 @@ List<DarwinArch> defaultIOSArchsForEnvironment(
   ];
 }
 
+// Returns the Dart SDK's name for the specified target architecture.
+//
+// When building for Darwin platforms, the tool invokes architecture-specific
+// variants of `gen_snapshot`, one for each target architecture. The output
+// instructions are then built into architecture-specific binaries, which are
+// merged into a universal binary using the `lipo` tool.
+String getDartNameForDarwinArch(DarwinArch arch) {
+  switch (arch) {
+    case DarwinArch.armv7:
+      return 'armv7';
+    case DarwinArch.arm64:
+      return 'arm64';
+    case DarwinArch.x86_64:
+      return 'x64';
+  }
+}
+
+// Returns Apple's name for the specified target architecture.
+//
+// When invoking Apple tools such as `xcodebuild` or `lipo`, the tool often
+// passes one or more target architectures as paramters. The names returned by
+// this function reflect Apple's name for the specified architecture.
+//
+// For consistency with developer expectations, Flutter outputs also use these
+// architecture names in its build products for Darwin target platforms.
 String getNameForDarwinArch(DarwinArch arch) {
   switch (arch) {
     case DarwinArch.armv7:

packages/flutter_tools/test/general.shard/build_info_test.dart

@@ -80,6 +80,18 @@ void main() {
     });
   });
 
+  testWithoutContext('getDartNameForDarwinArch returns name used in Dart SDK', () {
+    expect(getDartNameForDarwinArch(DarwinArch.armv7),  'armv7');
+    expect(getDartNameForDarwinArch(DarwinArch.arm64),  'arm64');
+    expect(getDartNameForDarwinArch(DarwinArch.x86_64), 'x64');
+  });
+
+  testWithoutContext('getNameForDarwinArch returns Apple names', () {
+    expect(getNameForDarwinArch(DarwinArch.armv7),  'armv7');
+    expect(getNameForDarwinArch(DarwinArch.arm64),  'arm64');
+    expect(getNameForDarwinArch(DarwinArch.x86_64), 'x86_64');
+  });
+
   testWithoutContext('getNameForTargetPlatform on Darwin arches', () {
     expect(getNameForTargetPlatform(TargetPlatform.ios, darwinArch: DarwinArch.arm64), 'ios-arm64');
     expect(getNameForTargetPlatform(TargetPlatform.ios, darwinArch: DarwinArch.armv7), 'ios-armv7');

packages/flutter_tools/test/general.shard/build_system/targets/macos_test.dart

@@ -384,15 +384,15 @@ void main() {
 
     processManager.addCommands(<FakeCommand>[
       FakeCommand(command: <String>[
-        'Artifact.genSnapshot.TargetPlatform.darwin.release',
+        'Artifact.genSnapshot.TargetPlatform.darwin.release_arm64',
         '--deterministic',
         '--snapshot_kind=app-aot-assembly',
         '--assembly=${environment.buildDir.childFile('arm64/snapshot_assembly.S').path}',
         '--strip',
         environment.buildDir.childFile('app.dill').path
       ]),
       FakeCommand(command: <String>[
-        'Artifact.genSnapshot.TargetPlatform.darwin.release',
+        'Artifact.genSnapshot.TargetPlatform.darwin.release_x64',
         '--deterministic',
         '--snapshot_kind=app-aot-assembly',
         '--assembly=${environment.buildDir.childFile('x86_64/snapshot_assembly.S').path}',