Update link for missing desktop projects (#76123)

packages/flutter_tools/lib/src/linux/build_linux.dart

@@ -39,7 +39,7 @@ Future<void> buildLinux(
   }) async {
   if (!linuxProject.cmakeFile.existsSync()) {
     throwToolExit('No Linux desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-app '
+      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Linux support to a project.');
   }
 

packages/flutter_tools/lib/src/macos/build_macos.dart

@@ -34,7 +34,7 @@ Future<void> buildMacOS({
 }) async {
   if (!flutterProject.macos.xcodeWorkspace.existsSync()) {
     throwToolExit('No macOS desktop project configured. '
-      'See https://flutter.dev/desktop#add-desktop-support-to-an-existing-app '
+      'See https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding macOS support to a project.');
   }
 

packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -35,7 +35,7 @@ Future<void> buildWindows(WindowsProject windowsProject, BuildInfo buildInfo, {
   if (!windowsProject.cmakeFile.existsSync()) {
     throwToolExit(
       'No Windows desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-app '
+      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Windows support to a project.');
   }
 

packages/flutter_tools/test/commands.shard/hermetic/build_linux_test.dart

@@ -115,7 +115,9 @@ void main() {
 
     expect(createTestCommandRunner(command).run(
       const <String>['build', 'linux', '--no-pub']
-    ), throwsToolExit(message: 'No Linux desktop project configured'));
+    ), throwsToolExit(message: 'No Linux desktop project configured. See '
+      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'to learn about adding Linux support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => linuxPlatform,
     FileSystem: () => fileSystem,

packages/flutter_tools/test/commands.shard/hermetic/build_macos_test.dart

@@ -118,7 +118,9 @@ void main() {
 
     expect(createTestCommandRunner(command).run(
       const <String>['build', 'macos', '--no-pub']
-    ), throwsToolExit(message: 'No macOS desktop project configured'));
+    ), throwsToolExit(message: 'No macOS desktop project configured. See '
+      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'to learn about adding macOS support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => macosPlatform,
     FileSystem: () => fileSystem,

packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart

@@ -138,7 +138,9 @@ void main() {
 
     expect(createTestCommandRunner(command).run(
       const <String>['windows', '--no-pub']
-    ), throwsToolExit(message: 'No Windows desktop project configured'));
+    ), throwsToolExit(message: 'No Windows desktop project configured. See '
+      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'to learn about adding Windows support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => windowsPlatform,
     FileSystem: () => fileSystem,