[flutter_tools] update message for `flutter create -t plugin` when no...

[flutter_tools] update message for `flutter create -t plugin` when no `--platforms` specified (#71962)

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

@@ -294,8 +294,9 @@ class CreateCommand extends CreateBase {
       if (!creatingNewProject && requestedPlatforms.isNotEmpty) {
         _printPluginUpdatePubspecMessage(relativePluginPath, platformsString);
       } else if (_getSupportedPlatformsInPlugin(projectDir).isEmpty){
-        globals.printError(_kNoPlatformsArgMessage);
+        _printNoPluginMessage();
       }
+      _printAddPlatformMessage(relativePluginPath);
     } else  {
       // Tell the user the next steps.
       final FlutterProject project = FlutterProject.fromPath(projectDirPath);
@@ -489,15 +490,22 @@ To edit platform code in an IDE see https://flutter.dev/developing-packages/#edi
 
 void _printPluginUpdatePubspecMessage(String pluginPath, String platformsString) {
   globals.printStatus('''
-
 You need to update $pluginPath/pubspec.yaml to support $platformsString.
-For more information, see https://flutter.dev/go/developing-plugins.
 
 ''', emphasis: true, color: TerminalColor.red);
 }
 
-const String _kNoPlatformsArgMessage = '''
+void _printNoPluginMessage() {
+    globals.printError('''
+You've created a plugin project that doesn't yet support any platforms.
+
+''');
+}
+
+void _printAddPlatformMessage(String pluginPath) {
+  globals.printStatus('''
+To add platforms, run `flutter create -t plugin --platforms <platforms> .` under $pluginPath.
+For more information, see https://flutter.dev/go/plugin-platforms.
 
-Must specify at least one platform using --platforms.
-For more information, see https://flutter.dev/go/developing-plugins.
-''';
+''');
+}

packages/flutter_tools/test/commands.shard/permeable/create_test.dart

@@ -32,7 +32,7 @@ import '../../src/context.dart';
 import '../../src/pubspec_schema.dart';
 import '../../src/testbed.dart';
 
-const String _kNoPlatformsMessage = 'Must specify at least one platform using --platforms.\n';
+const String _kNoPlatformsMessage = 'You\'ve created a plugin project that doesn\'t yet support any platforms.\n';
 const String frameworkRevision = '12345678';
 const String frameworkChannel = 'omega';
 // TODO(fujino): replace FakePlatform.fromPlatform() with FakePlatform()
@@ -2315,6 +2315,8 @@ void main() {
 
     await runner.run(<String>['create', '--no-pub', '--template=plugin', projectDir.path]);
     expect(logger.errorText, contains(_kNoPlatformsMessage));
+    expect(logger.statusText, contains('To add platforms, run `flutter create -t plugin --platforms <platforms> .` under ${globals.fs.path.normalize(globals.fs.path.relative(projectDir.path))}.'));
+    expect(logger.statusText, contains('For more information, see https://flutter.dev/go/plugin-platforms.'));
 
   }, overrides: <Type, Generator>{
     FeatureFlags: () => TestFeatureFlags(isLinuxEnabled: false),