Fix recently introduced strong-mode errors

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.services.correction.fix;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/plugin/edit/fix/fix_dart.dart';
@@ skipping 1852 matching lines @@
 ''');
     testFile = '/my/lib/test.dart';
     addTestSource(
         '''
 library my.lib;
 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]);
     // prepare fix
     testUnit = resolveLibraryUnit(testSource);
     AnalysisError error = _findErrorToFix();
     fix = await _assertHasFix(DartFixKind.CREATE_FILE, error);
     change = fix.change;
     // validate change
     List<SourceFileEdit> fileEdits = change.edits;
@@ skipping 3491 matching lines @@
   @override
   void t() { }
 }
 ''');
   }
 
   void verifyResult(String expectedResult) {
     expect(resultCode, expectedResult);
   }
 }