Add macOS to TargetPlatform (#43457)

This PR adds TargetPlatform.macOS to the TargetPlatform enum. This allows us to begin implementation of some adaptive UI based on which target platform is desired.

I haven't updated the tests here, that will come in a follow-up PR.

packages/flutter_tools/lib/src/commands/create.dart

diff --git a/packages/flutter_tools/lib/src/commands/create.dart b/packages/flutter_tools/lib/src/commands/create.dart
index 4fd4377ae..645c625 100644
--- a/packages/flutter_tools/lib/src/commands/create.dart
+++ b/packages/flutter_tools/lib/src/commands/create.dart
@@ -656,7 +656,8 @@
       // removed, and the new schema should always be used.
       'useNewPluginSchema': macos,
       // If a desktop platform is included, add a workaround for #31366.
-      'includeTargetPlatformWorkaround': macos,
+      // When Linux and Windows are added, we will need this workaround again.
+      'includeTargetPlatformWorkaround': false,
     };
   }
 

packages/flutter_tools/templates/app/lib/main.dart.tmpl

diff --git a/packages/flutter_tools/templates/app/lib/main.dart.tmpl b/packages/flutter_tools/templates/app/lib/main.dart.tmpl
index fa01186..40c8db1 100644
--- a/packages/flutter_tools/templates/app/lib/main.dart.tmpl
+++ b/packages/flutter_tools/templates/app/lib/main.dart.tmpl
@@ -18,7 +18,7 @@
 // Sets a platform override for desktop to avoid exceptions. See
 // https://flutter.dev/desktop#target-platform-override for more info.
 void _enablePlatformOverrideForDesktop() {
-  if (!kIsWeb && (Platform.isMacOS || Platform.isWindows || Platform.isLinux)) {
+  if (!kIsWeb && (Platform.isWindows || Platform.isLinux)) {
     debugDefaultTargetPlatformOverride = TargetPlatform.fuchsia;
   }
 }