Fix recently introduced strong-mode errors

pkg/analyzer/test/src/context/builder_test.dart

 // Copyright (c) 2016, 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.src.context.context_builder_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/plugin/options.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
@@ skipping 656 matching lines @@
   Uri _relativeUri(String path, {String from}) {
     String relativePath = pathContext.relative(path, from: from);
     return pathContext.toUri(relativePath);
   }
 }
 
 @reflectiveTest
 class EmbedderYamlLocatorTest extends EmbedderRelatedTest {
   void test_empty() {
     EmbedderYamlLocator locator = new EmbedderYamlLocator({
-      'fox': [pathTranslator.getResource(emptyPath)]
+      'fox': <Folder>[pathTranslator.getResource(emptyPath)]
     });
     expect(locator.embedderYamls, hasLength(0));
   }
 
   void test_invalid() {
     EmbedderYamlLocator locator = new EmbedderYamlLocator(null);
     locator.addEmbedderYaml(null, r'''{{{,{{}}},}}''');
     expect(locator.embedderYamls, hasLength(0));
   }
 
   void test_valid() {
     EmbedderYamlLocator locator = new EmbedderYamlLocator({
-      'fox': [pathTranslator.getResource(foxLib)]
+      'fox': <Folder>[pathTranslator.getResource(foxLib)]
     });
     expect(locator.embedderYamls, hasLength(1));
   }
 }
 
 class _TestOptionsProcessor implements OptionsProcessor {
   Map<String, Object> expectedOptions = null;
 
   int errorCount = 0;
 
   @override
   void onError(Exception exception) {
     errorCount++;
   }
 
   @override
   void optionsProcessed(AnalysisContext context, Map<String, Object> options) {
     if (expectedOptions == null) {
       fail('Unexpected invocation of optionsProcessed');
     }
     expect(options, hasLength(expectedOptions.length));
     for (String key in expectedOptions.keys) {
       expect(options.containsKey(key), isTrue, reason: 'missing key $key');
       expect(options[key], expectedOptions[key],
           reason: 'values for key $key do not match');
     }
   }
 }