Fix list syntax in python script.

tools/dom/scripts/htmldartgenerator.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
 from generator import AnalyzeOperation, ConstantOutputOrder, \
     DartDomNameOfAttribute, FindMatchingAttribute, IsDartCollectionType, \
     IsPureInterface, TypeOrNothing, GetAnnotationsAndComments, \
     FormatAnnotationsAndComments, ConvertToFuture, GetCallbackInfo
 from htmlrenamer import convert_to_future_members
 
 # Types that are accessible cross-frame in a limited fashion.
 # In these cases, the base type (e.g., WindowBase) provides restricted access
 # while the subtype (e.g., Window) provides full access to the
 # corresponding objects if there are from the same frame.
 _secure_base_types = {
   'Window': 'WindowBase',
   'Location': 'LocationBase',
   'History': 'HistoryBase',
 }
 
-_custom_factories = {
+_custom_factories = [
   'Notification',
   'EventSource',
-}
+]
 
 class HtmlDartGenerator(object):
   def __init__(self, interface, options):
     self._database = options.database
     self._interface = interface
     self._type_registry = options.type_registry
     self._interface_type_info = self._type_registry.TypeInfo(self._interface.id)
     self._renamer = options.renamer
     self._library_name = self._renamer.GetLibraryName(self._interface)
 
@@ skipping 611 matching lines @@
         walk(interface.parents)
       else:
         walk(interface.parents[1:])
     return result
 
   def _DartType(self, type_name):
     return self._type_registry.DartType(type_name)
 
   def _IsPrivate(self, name):
     return name.startswith('_')