Fix recently introduced strong-mode errors

pkg/analysis_server/test/services/refactoring/move_file_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.refactoring.move_files;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/refactoring/refactoring.dart';
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 93 matching lines @@
 import '22/new_name.dart';
 ''');
     assertNoFileChange(testFile);
   }
 
   test_file_importedLibrary_package() async {
     // configure packages
     testFile = '/packages/my_pkg/lib/aaa/test.dart';
     provider.newFile(testFile, '');
     Map<String, List<Folder>> packageMap = {
-      'my_pkg': [provider.getResource('/packages/my_pkg/lib')]
+      'my_pkg': <Folder>[provider.getResource('/packages/my_pkg/lib')]
     };
     context.sourceFactory = new SourceFactory([
       AbstractContextTest.SDK_RESOLVER,
       new PackageMapUriResolver(provider, packageMap),
       resourceResolver
     ]);
     // do testing
     String pathA = '/project/bin/a.dart';
     addSource(
         pathA,
@@ skipping 111 matching lines @@
     addSource(
         appPath,
         '''
 import 'package:testName/myLib.dart';
 export 'package:testName/myLib.dart';
 ''');
     // configure Uri resolves
     context.sourceFactory = new SourceFactory([
       AbstractContextTest.SDK_RESOLVER,
       new PackageMapUriResolver(provider, <String, List<Folder>>{
-        'testName': [provider.getResource('/testName/lib')]
+        'testName': <Folder>[provider.getResource('/testName/lib')]
       }),
       resourceResolver,
     ]);
     // analyze
     _performAnalysis();
     // perform refactoring
     refactoring = new MoveFileRefactoring(
         provider, searchEngine, context, null, '/testName');
     refactoring.newFile = '/newName';
     await _assertSuccessfulRefactoring();
@@ skipping 33 matching lines @@
         break;
       }
       for (ChangeNotice notice in result.changeNotices) {
         if (notice.source.fullName.startsWith('/project/')) {
           index.indexUnit(notice.resolvedDartUnit);
         }
       }
     }
   }
 }