Fix recently introduced strong-mode errors

pkg/analyzer/test/source/sdk_ext_test.dart

 // Copyright (c) 2015, 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 analyzer.test.source.sdk_ext_test;
 
+import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/sdk_ext.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../utils.dart';
 
 main() {
   initializeTestEnvironment();
   defineReflectiveTests(SdkExtUriResolverTest);
@@ skipping 20 matching lines @@
   }
 
   test_create_badJSON() {
     var resolver = new SdkExtUriResolver(null);
     resolver.addSdkExt(r'''{{{,{{}}},}}''', null);
     expect(resolver.length, equals(0));
   }
 
   test_create_noSdkExtPackageMap() {
     var resolver = new SdkExtUriResolver({
-      'fox': [resourceProvider.getResource('/empty')]
+      'fox': <Folder>[resourceProvider.getResource('/empty')]
     });
     expect(resolver.length, equals(0));
   }
 
   test_create_nullPackageMap() {
     var resolver = new SdkExtUriResolver(null);
     expect(resolver.length, equals(0));
   }
 
   test_create_sdkExtPackageMap() {
     var resolver = new SdkExtUriResolver({
-      'fox': [resourceProvider.getResource('/tmp')]
+      'fox': <Folder>[resourceProvider.getResource('/tmp')]
     });
     // We have four mappings.
     expect(resolver.length, equals(4));
     // Check that they map to the correct paths.
     expect(resolver['dart:fox'], equals("/tmp/slippy.dart"));
     expect(resolver['dart:bear'], equals("/tmp/grizzly.dart"));
     expect(resolver['dart:relative'], equals("/relative.dart"));
     expect(resolver['dart:deep'], equals("/tmp/deep/directory/file.dart"));
   }
 
   test_restoreAbsolute() {
     var resolver = new SdkExtUriResolver({
-      'fox': [resourceProvider.getResource('/tmp')]
+      'fox': <Folder>[resourceProvider.getResource('/tmp')]
     });
     var source = resolver.resolveAbsolute(Uri.parse('dart:fox'));
     expect(source, isNotNull);
     // Restore source's uri.
     var restoreUri = resolver.restoreAbsolute(source);
     expect(restoreUri, isNotNull);
     // Verify that it is 'dart:fox'.
     expect(restoreUri.toString(), equals('dart:fox'));
     expect(restoreUri.scheme, equals('dart'));
     expect(restoreUri.path, equals('fox'));
   }
 }