Hide internal methods like wrap_jso from the dart:html public interface

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

Index: tools/dom/templates/html/impl/impl_HTMLElement.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLElement.darttemplate b/tools/dom/templates/html/impl/impl_HTMLElement.darttemplate
index fdc305fd37bc4d832495e3ab84520a14fd083bf5..1d126319e53388315f106ecdf964f9255fe309a9 100644
--- a/tools/dom/templates/html/impl/impl_HTMLElement.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLElement.darttemplate
@@ -8,10 +8,17 @@ part of $LIBRARYNAME;
 $(ANNOTATIONS)$(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
+  // Flags to only try upgrading once. If there's a failure don't try upgrading
   // anymore.
   bool _badUpgradeOccurred = false;
-  bool get _isBadUpgrade => _badUpgradeOccurred;
+
+  /// Required for SDK Infrastructure. Internal use only.
+  ///
+  /// Did this encounter a failure attempting to upgrade to
+  /// a custom element.
+  @Deprecated("Required for SDK Infrastructure. Internal use only.")
+  bool get isBadUpgrade => _badUpgradeOccurred;
+
   void _badUpgrade() { _badUpgradeOccurred = true; }
 $endif
 }