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

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 
   @DomName('IDBObjectStore.add')
   Future add(value, [key]) {
@@ skipping 130 matching lines @@
       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/*<T>*/(sync: true);
 
     //TODO: Report stacktrace once issue 4061 is resolved.
     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 {
         controller.add(cursor);
         if (autoAdvance == true && controller.hasListener) {
           cursor.next();
         }
       }
     });
     return controller.stream;
   }
 }
+
+// ignore: STRONG_MODE_DOWN_CAST_COMPOSITE
+/*=To*/ _cast/*<To>*/(dynamic x) => x;