[flutter_tools] fix crash if the platform section was a list (#60927)

Missed validation to go with cast. Added test case to reproduce.

packages/flutter_tools/lib/src/plugins.dart

@@ -214,6 +214,10 @@ class Plugin {
     }
 
     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';
+        return <String>[errorMessage];
+      }
       return _validateMultiPlatformYaml(yaml['platforms'] as YamlMap);
     } else {
       return _validateLegacyYaml(yaml);

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

@@ -943,6 +943,26 @@ flutter:
                               <String, dynamic>{'pluginClass': 'SomeClass',
                                                 'package': 'com.example'});
   });
+
+  testWithoutContext('FlutterManifest validates a platform section that is a list '
+    'instead of a map', () {
+    const String manifest = '''
+name: test
+flutter:
+    plugin:
+      platforms:
+        - android
+''';
+    final BufferLogger logger = BufferLogger.test();
+    final FlutterManifest flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: logger,
+    );
+
+    expect(flutterManifest, null);
+    expect(logger.errorText,
+      contains('flutter.plugin.platforms should be a map with the platform name as the key'));
+  });
 }
 
 Matcher matchesManifest({