Disallow empty platforms and specific platforms when parsing manifest for plugins (#49262)

packages/flutter_tools/lib/src/plugins.dart

@@ -202,9 +202,12 @@ class Plugin {
 
   static List<String> _validateMultiPlatformYaml(YamlMap yaml) {
     bool isInvalid(String key, bool Function(YamlMap) validate) {
+      if (!yaml.containsKey(key)) {
+        return false;
+      }
       final dynamic value = yaml[key];
       if (value is! YamlMap) {
-        return false;
+        return true;
       }
       final YamlMap yamlValue = value as YamlMap;
       if (yamlValue.containsKey('default_package')) {
@@ -212,6 +215,10 @@ class Plugin {
       }
       return !validate(yamlValue);
     }
+
+    if (yaml == null) {
+      return <String>['Invalid "platforms" specification.'];
+    }
     final List<String> errors = <String>[];
     if (isInvalid(AndroidPlugin.kConfigKey, AndroidPlugin.validate)) {
       errors.add('Invalid "android" plugin specification.');

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

@@ -157,5 +157,27 @@ void main() {
 
       expect(plugin.platforms, <String, PluginPlatform> {});
     });
+
+    test('error on empty platforms', () {
+      const String pluginYamlRaw = 'platforms:\n';
+
+      final YamlMap pluginYaml = loadYaml(pluginYamlRaw) as YamlMap;
+      expect(
+            () => Plugin.fromYaml(_kTestPluginName, _kTestPluginPath, pluginYaml, const <String>[]),
+        throwsToolExit(message: 'Invalid "platforms" specification.'),
+      );
+    });
+
+    test('error on empty platform', () {
+      const String pluginYamlRaw =
+          'platforms:\n'
+          ' android:\n';
+
+      final YamlMap pluginYaml = loadYaml(pluginYamlRaw) as YamlMap;
+      expect(
+            () => Plugin.fromYaml(_kTestPluginName, _kTestPluginPath, pluginYaml, const <String>[]),
+        throwsToolExit(message: 'Invalid "android" plugin specification.'),
+      );
+    });
   });
 }