Fix recently introduced strong-mode errors

pkg/analysis_server/test/services/correction/fix_test.dart

Index: pkg/analysis_server/test/services/correction/fix_test.dart
diff --git a/pkg/analysis_server/test/services/correction/fix_test.dart b/pkg/analysis_server/test/services/correction/fix_test.dart
index 5f91052a44ace932710119c2575d1bc3c63f260c..60897d94f81436aebae6b50fa22ea3b6b52a3358 100644
--- a/pkg/analysis_server/test/services/correction/fix_test.dart
+++ b/pkg/analysis_server/test/services/correction/fix_test.dart
@@ -1870,7 +1870,7 @@ part 'my_part.dart';
         Uri.parse('package:my/test.dart'));
     // configure SourceFactory
     UriResolver pkgResolver = new PackageMapUriResolver(provider, {
-      'my': [provider.getResource('/my/lib')],
+      'my': <Folder>[provider.getResource('/my/lib')],
     });
     context.sourceFactory = new SourceFactory(
         [AbstractContextTest.SDK_RESOLVER, pkgResolver, resourceResolver]);