Allow unknown fields in pubspec plugin section (#45303)

packages/flutter_tools/lib/src/plugins.dart

@@ -151,15 +151,25 @@ class Plugin {
   }
 
   static List<String> validatePluginYaml(YamlMap yaml) {
-    if (yaml.containsKey('platforms')) {
-      final int numKeys = yaml.keys.toSet().length;
-      if (numKeys != 1) {
-        return <String>[
-          'Invalid plugin specification. There must be only one key: "platforms", found multiple: ${yaml.keys.join(',')}',
-        ];
-      } else {
-        return _validateMultiPlatformYaml(yaml['platforms'] as YamlMap);
+
+    final bool usesOldPluginFormat = const <String>{
+      'androidPackage',
+      'iosPrefix',
+      'pluginClass',
+    }.any(yaml.containsKey);
+
+    final bool usesNewPluginFormat = yaml.containsKey('platforms');
+
+    if (usesOldPluginFormat && usesNewPluginFormat) {
+      const String errorMessage =
+          'The flutter.plugin.platforms key cannot be used in combination with the old'
+          'flutter.plugin.{androidPackage,iosPrefix,pluginClass} keys.'
+          'See: https://flutter.dev/docs/development/packages-and-plugins/developing-packages#plugin';
+      return <String>[errorMessage];
     }
+
+    if (usesNewPluginFormat) {
+      return _validateMultiPlatformYaml(yaml['platforms'] as YamlMap);
     } else {
       return _validateLegacyYaml(yaml);
     }

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

@@ -78,5 +78,52 @@ void main() {
       expect(webPlugin.fileName, 'web_plugin.dart');
       expect(windowsPlugin.pluginClass, 'WinSamplePlugin');
     });
+
+    test('Unknown fields are allowed (allows some future compatibility)', () {
+      const String pluginYamlRaw = 'implements: same_plugin\n' // this should be ignored by the tool
+          'platforms:\n'
+          ' android:\n'
+          '  package: com.flutter.dev\n'
+          '  pluginClass: ASamplePlugin\n'
+          '  anUnknownField: ASamplePlugin\n' // this should be ignored by the tool
+          ' ios:\n'
+          '  pluginClass: ISamplePlugin\n'
+          ' linux:\n'
+          '  pluginClass: LSamplePlugin\n'
+          ' macos:\n'
+          '  pluginClass: MSamplePlugin\n'
+          ' web:\n'
+          '  pluginClass: WebSamplePlugin\n'
+          '  fileName: web_plugin.dart\n'
+          ' windows:\n'
+          '  pluginClass: WinSamplePlugin\n';
+
+      final dynamic pluginYaml = loadYaml(pluginYamlRaw);
+      final Plugin plugin =
+      Plugin.fromYaml(_kTestPluginName, _kTestPluginPath, pluginYaml);
+
+      final AndroidPlugin androidPlugin =
+      plugin.platforms[AndroidPlugin.kConfigKey];
+      final IOSPlugin iosPlugin = plugin.platforms[IOSPlugin.kConfigKey];
+      final LinuxPlugin linuxPlugin =
+      plugin.platforms[LinuxPlugin.kConfigKey];
+      final MacOSPlugin macOSPlugin =
+      plugin.platforms[MacOSPlugin.kConfigKey];
+      final WebPlugin webPlugin = plugin.platforms[WebPlugin.kConfigKey];
+      final WindowsPlugin windowsPlugin =
+      plugin.platforms[WindowsPlugin.kConfigKey];
+      final String androidPluginClass = androidPlugin.pluginClass;
+      final String iosPluginClass = iosPlugin.pluginClass;
+
+      expect(iosPluginClass, 'ISamplePlugin');
+      expect(androidPluginClass, 'ASamplePlugin');
+      expect(iosPlugin.classPrefix, '');
+      expect(androidPlugin.package, 'com.flutter.dev');
+      expect(linuxPlugin.pluginClass, 'LSamplePlugin');
+      expect(macOSPlugin.pluginClass, 'MSamplePlugin');
+      expect(webPlugin.pluginClass, 'WebSamplePlugin');
+      expect(webPlugin.fileName, 'web_plugin.dart');
+      expect(windowsPlugin.pluginClass, 'WinSamplePlugin');
+    });
   });
 }