Hide the accessors for bad custom element upgrade

tools/dom/templates/html/impl/impl_HTMLElement.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;
 
 @DocsEditable()
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
 $if DARTIUM
   // Flags to only try upgrading once if there's a failure don't try upgrading
   // anymore.
-  bool _badUpgrade = false;
-  bool get isBadUpgrade => _badUpgrade;
-  void badUpgrade() { _badUpgrade = true; }
+  bool _badUpgradeOccurred = false;
+  bool get _isBadUpgrade => _badUpgradeOccurred;
+  void _badUpgrade() { _badUpgradeOccurred = true; }
 $endif
 }