Fix some type narrowing

lib/html/scripts/systemhtml.py

 #!/usr/bin/python
 # 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.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 import os
@@ skipping 1039 matching lines @@
 
   def _NarrowToImplementationType(self, type_name):
     if type_name == 'Dynamic':
       return type_name
     return self._type_registry.TypeInfo(type_name).narrow_dart_type()
 
   def _NarrowInputType(self, type_name):
     return self._NarrowToImplementationType(type_name)
 
   def _NarrowOutputType(self, type_name):
-    secure_name = SecureOutputType(self, type_name)
-    if (type_name == secure_name):
-      return self._NarrowToImplementationType(type_name)
-    else:
-      return secure_name
+    secure_name = SecureOutputType(self, type_name, True)
+    return self._NarrowToImplementationType(secure_name)
 
   def _FindShadowedAttribute(self, attr, merged_interfaces={}):
     """Returns (attribute, superinterface) or (None, None)."""
     def FindInParent(interface):
       """Returns matching attribute in parent, or None."""
       if interface.parents:
         parent = interface.parents[0]
         if IsDartCollectionType(parent.type.id):
           return (None, None)
         if IsPureInterface(parent.type.id):
@@ skipping 54 matching lines @@
 
     library_emitter = self._multiemitter.FileEmitter(library_file_path)
     library_file_dir = os.path.dirname(library_file_path)
     auxiliary_dir = os.path.relpath(auxiliary_dir, library_file_dir)
     imports_emitter = library_emitter.Emit(
         self._template, AUXILIARY_DIR=massage_path(auxiliary_dir))
     for path in sorted(self._path_to_emitter.keys()):
       relpath = os.path.relpath(path, library_file_dir)
       imports_emitter.Emit(
           "#source('$PATH');\n", PATH=massage_path(relpath))