dart2js_html: Fix for issue 29538 - some returned lists may be null

sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart

Index: sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart
diff --git a/sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart b/sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart
index 1d72b124a388c1df9c1c7e8bc5c1ec4c9540ee95..9f49fe193da80e44ee9d92113144fdb520c80532 100644
--- a/sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart
+++ b/sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart
@@ -449,7 +449,7 @@ class Database extends EventTarget {
 
   @DomName('IDBDatabase.objectStoreNames')
   @DocsEditable()
-  @Returns('DomStringList')
+  @Returns('DomStringList|Null')
   @Creates('DomStringList')
   final List<String> objectStoreNames;
 
@@ -1036,7 +1036,7 @@ class ObjectStore extends Interceptor {
 
   @DomName('IDBObjectStore.indexNames')
   @DocsEditable()
-  @Returns('DomStringList')
+  @Returns('DomStringList|Null')
   @Creates('DomStringList')
   final List<String> indexNames;
 
@@ -1424,7 +1424,7 @@ class Transaction extends EventTarget {
   @DomName('IDBTransaction.objectStoreNames')
   @DocsEditable()
   @Experimental() // untriaged
-  @Returns('DomStringList')
+  @Returns('DomStringList|Null')
   @Creates('DomStringList')
   final List<String> objectStoreNames;