Reapply the fix.

lib/dom/scripts/systemnative.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 systems to generate
 native binding from the IDL database."""
 
 import emitter
 import os
@@ skipping 787 matching lines @@
           INDEX=len(arguments) + 1)
 
     # Emit arguments.
     start_index = 1 if needs_receiver else 0
     for i, argument in enumerate(arguments):
       type_info = self._TypeInfo(argument.type.id)
       self._cpp_impl_includes |= set(type_info.to_native_includes())
       cpp_arguments.append(type_info.emit_to_native(
           body_emitter,
           argument,
-          IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument),
+          (IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument)) or (argument.ext_attrs.get('Optional') == 'DefaultIsNullString'),
           # TODO(antonm): all IDs should be renamed when database is generated.
           # That will allow to get rid of argument_name argument altogether.
           DartDomNameOfAttribute(argument),
           'Dart_GetNativeArgument(args, %i)' % (start_index + i),
           self._interface.id))
 
     body_emitter.Emit('\n')
 
     if 'NeedsUserGestureCheck' in ext_attrs:
       cpp_arguments.append('DartUtilities::processingUserGesture');
@@ skipping 101 matching lines @@
     parent_interface = _FindInHierarchy(database, parent_interface, test)
     if parent_interface:
       return parent_interface
 
 def _ToWebKitName(name):
   name = name[0].lower() + name[1:]
   name = re.sub(r'^(hTML|uRL|jS|xML|xSLT)', lambda s: s.group(1).lower(),
                 name)
   return re.sub(r'^(create|exclusive)', lambda s: 'is' + s.group(1).capitalize(),
                 name)