IDL: Add build target for IDL dictionary impl generation in core

Source/bindings/scripts/v8_methods.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 205 matching lines @@
     this_cpp_value = cpp_value(interface, method, index)
     is_variadic_wrapper_type = argument.is_variadic and idl_type.is_wrapper_type
     return_promise = (method.idl_type.name == 'Promise' if method.idl_type
                                                         else False)
 
     if ('ImplementedInPrivateScript' in extended_attributes and
         not idl_type.is_wrapper_type and
         not idl_type.is_basic_type):
         raise Exception('Private scripts supports only primitive types and DOM wrappers.')
 
+    default_cpp_value = argument.default_cpp_value
     return {
         'cpp_type': idl_type.cpp_type_args(extended_attributes=extended_attributes,
                                            raw_type=True,
                                            used_as_variadic_argument=argument.is_variadic),
         'cpp_value': this_cpp_value,
         # FIXME: check that the default value's type is compatible with the argument's
-        'default_value': argument.default_cpp_value,
+        'default_value': default_cpp_value,
         'enum_validation_expression': idl_type.enum_validation_expression,
         'handle': '%sHandle' % argument.name,
         # FIXME: remove once [Default] removed and just use argument.default_value
-        'has_default': 'Default' in extended_attributes or argument.default_value,
+        'has_default': 'Default' in extended_attributes or default_cpp_value,
         'has_type_checking_interface':
             (has_extended_attribute_value(interface, 'TypeChecking', 'Interface') or
              has_extended_attribute_value(method, 'TypeChecking', 'Interface')) and
             idl_type.is_wrapper_type,
         'has_type_checking_unrestricted':
             (has_extended_attribute_value(interface, 'TypeChecking', 'Unrestricted') or
              has_extended_attribute_value(method, 'TypeChecking', 'Unrestricted')) and
             idl_type.name in ('Float', 'Double'),
         # Dictionary is special-cased, but arrays and sequences shouldn't be
         'idl_type': idl_type.base_type,
@@ skipping 192 matching lines @@
     }
 
 
 def union_arguments(idl_type):
     return [union_member_argument_context(member_idl_type, index)
             for index, member_idl_type
             in enumerate(idl_type.member_types)]
 
 
 def argument_default_cpp_value(argument):
+    if argument.idl_type.is_dictionary:
+        # We always create impl objects for IDL dictionaries.
+        return '%s::create()' % argument.idl_type.base_type
     if not argument.default_value:
         return None
     return argument.idl_type.literal_cpp_value(argument.default_value)
 
 IdlTypeBase.union_arguments = None
 IdlUnionType.union_arguments = property(union_arguments)
 IdlArgument.default_cpp_value = property(argument_default_cpp_value)