Allow null assignments if there is a corresponding marker.

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 714 matching lines @@
           '            return;\n',
           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)
       argument_expression_template, type, cls, function = \
           type_info.to_native_info(argument, self._interface.id)
 
-      if (isinstance(argument, IDLArgument) and (
-          (IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument)) or
-           (argument.ext_attrs.get('Default') == 'NullString') or
-           _IsOptionalStringArgumentInInitEventMethod(self._interface, node, argument))):
+      def AllowsNull():
+        assert argument.ext_attrs.get('TreatNullAs', 'NullString') == 'NullString'
+        if argument.ext_attrs.get('TreatNullAs') == 'NullString':
+          return True
+
+        if isinstance(argument, IDLArgument):
+          if IsOptional(argument) and not self._IsArgumentOptionalInWebCore(node, argument):
+            return True
+          if argument.ext_attrs.get('Default') == 'NullString':
+            return True
+          if _IsOptionalStringArgumentInInitEventMethod(self._interface, node, argument):
+            return True
+
+        return False
+
+      if AllowsNull():
         function += 'WithNullCheck'
 
       argument_name = DartDomNameOfAttribute(argument)
       if type_info.pass_native_by_ref():
         invocation_template =\
             '        $TYPE $ARGUMENT_NAME;\n'\
             '        $CLS::$FUNCTION(Dart_GetNativeArgument(args, $INDEX), $ARGUMENT_NAME, exception);\n'
       else:
         invocation_template =\
             '        $TYPE $ARGUMENT_NAME = $CLS::$FUNCTION(Dart_GetNativeArgument(args, $INDEX), exception);\n'
@@ skipping 181 matching lines @@
             '    if (Dart_NativeFunction func = $CLASS_NAME::resolver(name, argumentCount))\n'
             '        return func;\n',
             CLASS_NAME=os.path.splitext(os.path.basename(path))[0])
 
 def _IsOptionalStringArgumentInInitEventMethod(interface, operation, argument):
   return (
       interface.id.endswith('Event') and
       operation.id.startswith('init') and
       argument.ext_attrs.get('Default') == 'Undefined' and
       argument.type.id == 'DOMString')