IDL: Use Nullable for union type return value

Source/bindings/scripts/v8_types.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 701 matching lines @@
     return statement
 
 
 def v8_set_return_value_union(idl_type, cpp_value, extended_attributes=None, script_wrappable='', release=False, for_main_world=False):
     """
     release: can be either False (False for all member types) or
              a sequence (list or tuple) of booleans (if specified individually).
     """
 
     return [
-        member_type.v8_set_return_value(cpp_value + str(i),
+        member_type.v8_set_return_value(cpp_value.format(index=i),
                                         extended_attributes,
                                         script_wrappable,
                                         release and release[i],
                                         for_main_world)
             for i, member_type in
             enumerate(idl_type.member_types)]
 
 IdlType.v8_set_return_value = v8_set_return_value
 IdlUnionType.v8_set_return_value = v8_set_return_value_union
 
@@ skipping 71 matching lines @@
 
 def is_explicit_nullable(idl_type):
     # Nullable type that isn't implicit nullable (see above.) For such types,
     # we use Nullable<T> or similar explicit ways to represent a null value.
     return idl_type.is_nullable and not idl_type.is_implicit_nullable
 
 IdlType.is_implicit_nullable = property(is_implicit_nullable)
 IdlType.is_explicit_nullable = property(is_explicit_nullable)
 IdlUnionType.is_implicit_nullable = False
 IdlUnionType.is_explicit_nullable = property(is_explicit_nullable)