Fix indexed_db strong mode warnings.

tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate

Index: tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate b/tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate
index 1363056fb541de952d3f34c08dda634760515524..46bb7dd06c8eced1162f88ce98033a1ec54ddb10 100644
--- a/tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate
+++ b/tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate
@@ -137,18 +137,18 @@ $!MEMBERS
   /**
    * Helper for iterating over cursors in a request.
    */
-  static Stream<Cursor> _cursorStreamFromResult(Request request,
+  static Stream/*<T>*/ _cursorStreamFromResult/*<T extends Cursor>*/(Request request,
       bool autoAdvance) {
     // TODO: need to guarantee that the controller provides the values
     // immediately as waiting until the next tick will cause the transaction to
     // close.
-    var controller = new StreamController(sync: true);
+    var controller = new StreamController/*<T>*/(sync: true);
 
     //TODO: Report stacktrace once issue 4061 is resolved.
     request.onError.listen(controller.addError);
 
     request.onSuccess.listen((e) {
-      Cursor cursor = request.result;
+      var cursor = request.result as dynamic /*=T*/;
       if (cursor == null) {
         controller.close();
       } else {