Making Indexed DB's cursor stream not auto-advance if no subscribers

sdk/lib/indexed_db/dartium/indexed_db_dartium.dart

Index: sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
diff --git a/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart b/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
index c0d888cbc4cb8761ecc3c266314bc56157af3269..f902887aea0fe42dcf278fda90c5f27ccb8e953d 100644
--- a/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
+++ b/sdk/lib/indexed_db/dartium/indexed_db_dartium.dart
@@ -1073,7 +1073,7 @@ class ObjectStore extends NativeFieldWrapperClass1 {
         controller.close();
       } else {
         controller.add(cursor);
-        if (autoAdvance == true) {
+        if (autoAdvance == true && controller.hasSubscribers) {
           cursor.next();
         }
       }