Use proper default values for flags.

tools/dom/scripts/generator.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 systems to generate
 Dart APIs from the IDL database."""
 
 import copy
 import re
@@ skipping 1050 matching lines @@
     conversion_cast = conversion_cast % (self.native_type(), value)
     return 'Dart%s::toDart(%s)' % (self._idl_type, conversion_cast)
 
   def argument_expression(self, name, interface_name):
     return name if interface_name.endswith('List') else '%s->propertyReference()' % name
 
 
 class TypeData(object):
   def __init__(self, clazz, dart_type=None, native_type=None,
                merged_interface=None, merged_into=None,
-               custom_to_dart=None, custom_to_native=None,
+               custom_to_dart=False, custom_to_native=False,
                conversion_includes=None,
                webcore_getter_name='getAttribute',
                webcore_setter_name='setAttribute',
                item_type=None, suppress_interface=False, is_typed_array=False):
     self.clazz = clazz
     self.dart_type = dart_type
     self.native_type = native_type
     self.merged_interface = merged_interface
     self.merged_into = merged_into
     self.custom_to_dart = custom_to_dart
@@ skipping 202 matching lines @@
                                   self)
 
     if type_data.clazz == 'SVGTearOff':
       dart_interface_name = self._renamer.RenameInterface(
           self._database.GetInterface(type_name))
       return SVGTearOffIDLTypeInfo(
           type_name, type_data, dart_interface_name, self)
 
     class_name = '%sIDLTypeInfo' % type_data.clazz
     return globals()[class_name](type_name, type_data)