Fix recently introduced strong-mode errors

pkg/analysis_server/test/domain_analysis_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.domain.analysis;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/domain_analysis.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
+import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:path/path.dart';
 import 'package:plugin/manager.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import 'analysis_abstract.dart';
 import 'mock_sdk.dart';
@@ skipping 378 matching lines @@
 ''');
     String pkgDependency = posix.join(projectPath, 'package_dep');
     resourceProvider.newFile(pkgDependency, 'contents');
     packageMapProvider.dependencies.add(pkgDependency);
     // Create project and wait for analysis
     createProject();
     await waitForTasksFinished();
     expect(filesErrors[testFile], isNotEmpty);
     // Add the package to the package map and tickle the package dependency.
     packageMapProvider.packageMap = {
-      'pkgA': [resourceProvider.getResource('/packages/pkgA')]
+      'pkgA': <Folder>[resourceProvider.getResource('/packages/pkgA')]
     };
     resourceProvider.modifyFile(pkgDependency, 'new contents');
     // Give the server time to notice the file has changed, then let
     // analysis complete. There should now be no error.
     await pumpEventQueue();
     await waitForTasksFinished();
     expect(filesErrors[testFile], isEmpty);
   }
 
   test_setRoots_packages() {
@@ skipping 386 matching lines @@
   test_beforeAnalysis() async {
     addTestFile('int V = 42;');
     createProject();
     // subscribe
     addAnalysisSubscription(AnalysisService.HIGHLIGHTS, testFile);
     // wait for analysis
     await waitForTasksFinished();
     expect(filesHighlights[testFile], isNotEmpty);
   }
 }