Fix recently introduced strong-mode errors

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

Index: pkg/analyzer/test/src/context/builder_test.dart
diff --git a/pkg/analyzer/test/src/context/builder_test.dart b/pkg/analyzer/test/src/context/builder_test.dart
index 5fae924932ef562b223b03d6bfb44021aba8d078..5f04b4f97a51cf3d935aec637ac36b3acce654ac 100644
--- a/pkg/analyzer/test/src/context/builder_test.dart
+++ b/pkg/analyzer/test/src/context/builder_test.dart
@@ -674,7 +674,7 @@ analyzer:
 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));
   }
@@ -687,7 +687,7 @@ class EmbedderYamlLocatorTest extends EmbedderRelatedTest {
 
   void test_valid() {
     EmbedderYamlLocator locator = new EmbedderYamlLocator({
-      'fox': [pathTranslator.getResource(foxLib)]
+      'fox': <Folder>[pathTranslator.getResource(foxLib)]
     });
     expect(locator.embedderYamls, hasLength(1));
   }