Fix recently introduced strong-mode errors

pkg/analysis_server/test/context_manager_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.context.directory.manager;
 
 import 'dart:collection';
 import 'dart:io' as io;
 
 import 'package:analysis_server/src/context_manager.dart';
@@ skipping 427 matching lines @@
 
   void test_setRoots_addFolderWithNestedPackageSpec() {
     String examplePath = newFolder([projPath, ContextManagerTest.EXAMPLE_NAME]);
     String libPath = newFolder([projPath, ContextManagerTest.LIB_NAME]);
 
     newFile([projPath, ContextManagerImpl.PACKAGE_SPEC_NAME]);
     newFile([libPath, 'main.dart']);
     newFile([examplePath, ContextManagerImpl.PACKAGE_SPEC_NAME]);
     newFile([examplePath, 'example.dart']);
 
-    packageMapProvider.packageMap['proj'] = [
+    packageMapProvider.packageMap['proj'] = <Folder>[
       resourceProvider.getResource(libPath)
     ];
 
     manager.setRoots(<String>[projPath], <String>[], <String, String>{});
 
     expect(callbacks.currentContextPaths, hasLength(2));
 
     expect(callbacks.currentContextPaths, contains(projPath));
     Set<Source> projSources = callbacks.currentContextSources[projPath];
     expect(projSources, hasLength(1));
     expect(projSources.first.uri.toString(), 'file:///my/proj/lib/main.dart');
 
     expect(callbacks.currentContextPaths, contains(examplePath));
     Set<Source> exampleSources = callbacks.currentContextSources[examplePath];
     expect(exampleSources, hasLength(1));
     expect(exampleSources.first.uri.toString(),
         'file:///my/proj/example/example.dart');
   }
 
   void test_setRoots_addFolderWithNestedPubspec() {
     String examplePath = newFolder([projPath, ContextManagerTest.EXAMPLE_NAME]);
     String libPath = newFolder([projPath, ContextManagerTest.LIB_NAME]);
 
     newFile([projPath, ContextManagerImpl.PUBSPEC_NAME]);
     newFile([libPath, 'main.dart']);
     newFile([examplePath, ContextManagerImpl.PUBSPEC_NAME]);
     newFile([examplePath, 'example.dart']);
 
-    packageMapProvider.packageMap['proj'] = [
+    packageMapProvider.packageMap['proj'] = <Folder>[
       resourceProvider.getResource(libPath)
     ];
 
     manager.setRoots(<String>[projPath], <String>[], <String, String>{});
 
     expect(callbacks.currentContextPaths, hasLength(2));
 
     expect(callbacks.currentContextPaths, contains(projPath));
     Set<Source> projSources = callbacks.currentContextSources[projPath];
     expect(projSources, hasLength(1));
@@ skipping 78 matching lines @@
     String libPath = newFolder([projPath, ContextManagerTest.LIB_NAME]);
     String srcPath = newFolder([libPath, ContextManagerTest.SRC_NAME]);
     String testPath = newFolder([projPath, ContextManagerTest.TEST_NAME]);
 
     newFile([projPath, ContextManagerImpl.PUBSPEC_NAME]);
     String appPath = newFile([binPath, 'app.dart']);
     newFile([libPath, 'main.dart']);
     newFile([srcPath, 'internal.dart']);
     String testFilePath = newFile([testPath, 'main_test.dart']);
 
-    packageMapProvider.packageMap['proj'] = [
+    packageMapProvider.packageMap['proj'] = <Folder>[
       resourceProvider.getResource(libPath)
     ];
 
     manager.setRoots(<String>[projPath], <String>[], <String, String>{});
     Set<Source> sources = callbacks.currentContextSources[projPath];
 
     expect(callbacks.currentContextPaths, hasLength(1));
     expect(callbacks.currentContextPaths, contains(projPath));
     expect(sources, hasLength(4));
     List<String> uris =
@@ skipping 2179 matching lines @@
 class TestUriResolver extends UriResolver {
   Map<Uri, Source> uriMap;
 
   TestUriResolver(this.uriMap);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     return uriMap[uri];
   }
 }