Fix handling default parameters

tools/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 1532 matching lines @@
         if argument.type.nullable:
           return True
 
         if isinstance(argument, IDLAttribute):
           return (argument.type.id == 'DOMString') and \
               ('Reflect' in argument.ext_attrs)
 
         if isinstance(argument, IDLArgument):
           if IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument):
             return True
-          if argument.ext_attrs.get('Default') == 'NullString':
+          # argument default to null (e.g., DOMString arg = null).
+          if argument.default_value_is_null:
             return True
           if _IsOptionalStringArgumentInInitEventMethod(self._interface, node, argument):
             return True
 
         return False
 
       if AllowsNull():
         function += 'WithNullCheck'
 
       argument_name = DartDomNameOfAttribute(argument)
@@ skipping 218 matching lines @@
     else:
       return '%s::%s' % (impl_type_name, function_name)
 
   def _IsArgumentOptionalInWebCore(self, operation, argument):
     if not IsOptional(argument):
       return False
     if 'Callback' in argument.ext_attrs:
       return False
     if operation.id in ['addEventListener', 'removeEventListener'] and argument.id == 'useCapture':
       return False
-    if 'ForceOptional' in argument.ext_attrs:
+    if 'DartForceOptional' in argument.ext_attrs:
       return False
     if argument.type.id == 'Dictionary':
       return False
     return True
 
   def _GenerateCPPIncludes(self, includes):
     return ''.join(['#include %s\n' % include for include in sorted(includes)])
 
   def _ToWebKitName(self, name):
     name = name[0].lower() + name[1:]
@@ skipping 187 matching lines @@
       e.Emit('    },\n')
 
     e.Emit("};\n");
     e.Emit('\n');
     e.Emit('} // namespace WebCore\n');
 
 def _IsOptionalStringArgumentInInitEventMethod(interface, operation, argument):
   return (
       interface.id.endswith('Event') and
       operation.id.startswith('init') and
-      argument.ext_attrs.get('Default') == 'Undefined' and
+      argument.default_value == 'Undefined' and
       argument.type.id == 'DOMString')