Fix the message strings for xcodeMissing and xcodeIncomplete (#117922)

* Add macOS to xcodeMissing and xcodeIncomplete

* And unit test

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

@@ -176,10 +176,10 @@ class UserMessages {
       'Launch Xcode and install additional required components when prompted or run:\n'
       '  sudo xcodebuild -runFirstLaunch';
   String get xcodeMissing =>
-      'Xcode not installed; this is necessary for iOS development.\n'
+      'Xcode not installed; this is necessary for iOS and macOS development.\n'
       'Download at https://developer.apple.com/xcode/download/.';
   String get xcodeIncomplete =>
-      'Xcode installation is incomplete; a full installation is necessary for iOS development.\n'
+      'Xcode installation is incomplete; a full installation is necessary for iOS and macOS development.\n'
       'Download at: https://developer.apple.com/xcode/download/\n'
       'Or install Xcode via the App Store.\n'
       'Once installed, run:\n'

packages/flutter_tools/test/general.shard/base/user_messages_test.dart

@@ -30,4 +30,10 @@ void main() {
     checkInstallationURL((Platform platform) => userMessages.androidSdkBuildToolsOutdated(0, '', platform));
     checkInstallationURL((Platform platform) => userMessages.androidStudioInstallation(platform));
   });
+
+  testWithoutContext('Xcode installation instructions', () {
+    final UserMessages userMessages = UserMessages();
+    expect(userMessages.xcodeMissing, contains('iOS and macOS'));
+    expect(userMessages.xcodeIncomplete, contains('iOS and macOS'));
+  });
 }