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

tools/dom/templates/html/impl/impl_IDBFactory.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 {
   /**
    * Checks to see if Indexed DB is supported on the current platform.
    */
@@ skipping 87 matching lines @@
 
 /**
  * Ties a request to a completer, so the completer is completed when it succeeds
  * and errors out when the request errors.
  */
 Future/*<T>*/ _completeRequest/*<T>*/(Request request) {
   var completer = new Completer/*<T>*/.sync();
   // TODO: make sure that completer.complete is synchronous as transactions
   // may be committed if the result is not processed immediately.
   request.onSuccess.listen((e) {
-    completer.complete(request.result as dynamic/*=T*/);
+    var result = _cast/*<T>*/(request.result);
+    completer.complete(result);
   });
   request.onError.listen(completer.completeError);
   return completer.future;
 }