Use _downcast and _cast instead of "as" to reduce dart2js output size.

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 46bb7dd06c8eced1162f88ce98033a1ec54ddb10..4c74806c134bf8108a59e35ebc130d8350576c6c 100644
--- a/tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate
+++ b/tools/dom/templates/html/impl/impl_IDBObjectStore.darttemplate
@@ -148,7 +148,7 @@ $!MEMBERS
     request.onError.listen(controller.addError);
 
     request.onSuccess.listen((e) {
-      var cursor = request.result as dynamic /*=T*/;
+      var cursor = _cast/*<T>*/(request.result);
       if (cursor == null) {
         controller.close();
       } else {
@@ -161,3 +161,6 @@ $!MEMBERS
     return controller.stream;
   }
 }
+
+// ignore: STRONG_MODE_DOWN_CAST_COMPOSITE
+/*=To*/ _cast/*<To>*/(dynamic x) => x;