Show correct errors when plugins yaml forgot the 'flutter.plugins.pla… (#61338)

packages/flutter_tools/lib/src/plugins.dart

@@ -213,6 +213,14 @@ class Plugin {
       return <String>[errorMessage];
     }
 
+    if (!usesOldPluginFormat && !usesNewPluginFormat) {
+      const String errorMessage =
+          'Cannot find the `flutter.plugin.platforms` key in the `pubspec.yaml` file. '
+          'An instruction to format the `pubspec.yaml` can be found here: '
+          'https://flutter.dev/docs/development/packages-and-plugins/developing-packages#plugin-platforms';
+      return <String>[errorMessage];
+    }
+
     if (usesNewPluginFormat) {
       if (yaml['platforms'] != null && yaml['platforms'] is! YamlMap) {
         const String errorMessage = 'flutter.plugin.platforms should be a map with the platform name as the key';
@@ -264,6 +272,7 @@ class Plugin {
 
   static List<String> _validateLegacyYaml(YamlMap yaml) {
     final List<String> errors = <String>[];
+
     if (yaml['androidPackage'] != null && yaml['androidPackage'] is! String) {
       errors.add('The "androidPackage" must either be null or a string.');
     }

packages/flutter_tools/test/general.shard/flutter_manifest_test.dart

@@ -1017,6 +1017,28 @@ flutter:
     expect(logger.errorText,
       contains('flutter.plugin.platforms should be a map with the platform name as the key'));
   });
+
+    testWithoutContext('FlutterManifest validates plugin format not support.', () {
+    const String manifest = '''
+name: test
+flutter:
+  plugin:
+    android:
+      package: com.example
+      pluginClass: SomeClass
+    ios:
+      pluginClass: SomeClass
+''';
+    final BufferLogger logger = BufferLogger.test();
+    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: logger,
+    );
+
+    expect(flutterManifest, null);
+    expect(logger.errorText,
+      contains('Cannot find the `flutter.plugin.platforms` key in the `pubspec.yaml` file. '));
+  });
 }
 
 Matcher matchesManifest({