Default `NativeAssets` Darwin and IOS target archs if missing (#136948)

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

Bug:

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

* https://github.com/flutter/flutter/pull/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.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [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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packages/flutter_tools/lib/src/build_info.dart

@@ -920,7 +920,7 @@ const String kIosArchs = 'IosArchs';
 /// The define to control what macOS architectures are built for.
 ///
 /// This is expected to be a space-delimited list of architectures. If not
-/// provided, defaults to x86_64.
+/// provided, defaults to x86_64 and arm64.
 ///
 /// Supported values are x86_64 and arm64.
 const String kDarwinArchs = 'DarwinArchs';

packages/flutter_tools/lib/src/build_system/targets/native_assets.dart

@@ -81,11 +81,12 @@ class NativeAssets extends Target {
 
       switch (targetPlatform) {
         case TargetPlatform.ios:
-          final String? iosArchsEnvironment = environment.defines[kIosArchs];
-          if (iosArchsEnvironment == null) {
-            throw MissingDefineException(kIosArchs, name);
-          }
-          final List<DarwinArch> iosArchs = iosArchsEnvironment.split(' ').map(getDarwinArchForName).toList();
+          final List<DarwinArch> iosArchs =
+            _emptyToNull(environment.defines[kIosArchs])
+            ?.split(' ')
+            .map(getIOSArchForName)
+            .toList()
+            ?? <DarwinArch>[DarwinArch.arm64];
           final String? environmentBuildMode = environment.defines[kBuildMode];
           if (environmentBuildMode == null) {
             throw MissingDefineException(kBuildMode, name);
@@ -107,11 +108,12 @@ class NativeAssets extends Target {
             yamlParentDirectory: environment.buildDir.uri,
           );
         case TargetPlatform.darwin:
-          final String? darwinArchsEnvironment = environment.defines[kDarwinArchs];
-          if (darwinArchsEnvironment == null) {
-            throw MissingDefineException(kDarwinArchs, name);
-          }
-          final List<DarwinArch> darwinArchs = darwinArchsEnvironment.split(' ').map(getDarwinArchForName).toList();
+          final List<DarwinArch> darwinArchs =
+            _emptyToNull(environment.defines[kDarwinArchs])
+            ?.split(' ')
+            .map(getDarwinArchForName)
+            .toList()
+            ?? <DarwinArch>[DarwinArch.x86_64, DarwinArch.arm64];
           final String? environmentBuildMode = environment.defines[kBuildMode];
           if (environmentBuildMode == null) {
             throw MissingDefineException(kBuildMode, name);
@@ -249,3 +251,10 @@ class NativeAssets extends Target {
     Source.pattern('{BUILD_DIR}/native_assets.yaml'),
   ];
 }
+
+String? _emptyToNull(String? input) {
+  if (input == null || input.isEmpty) {
+    return null;
+  }
+  return input;
+}

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

@@ -55,11 +55,19 @@ void main() {
     expect(const NativeAssets().build(iosEnvironment), throwsA(isA<MissingDefineException>()));
   });
 
-  testUsingContext('NativeAssets throws error if missing ios archs', () async {
+  testUsingContext('NativeAssets defaults to ios archs if missing', () async {
     await createPackageConfig(iosEnvironment);
 
     iosEnvironment.defines.remove(kIosArchs);
-    expect(const NativeAssets().build(iosEnvironment), throwsA(isA<MissingDefineException>()));
+
+    final NativeAssetsBuildRunner buildRunner = FakeNativeAssetsBuildRunner();
+    await NativeAssets(buildRunner: buildRunner).build(iosEnvironment);
+
+    final File nativeAssetsYaml =
+        iosEnvironment.buildDir.childFile('native_assets.yaml');
+    final File depsFile = iosEnvironment.buildDir.childFile('native_assets.d');
+    expect(depsFile, exists);
+    expect(nativeAssetsYaml, exists);
   });
 
   testUsingContext('NativeAssets throws error if missing sdk root', () async {