IDL: Avoid double type-check for variadic interface type arguments

Source/bindings/templates/methods.cpp

 {##############################################################################}
 {% macro generate_method(method, world_suffix) %}
 {% filter conditional(method.conditional_string) %}
 static void {{method.name}}{{method.overload_index}}Method{{world_suffix}}(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     {# Local variables #}
     {% if method.has_exception_state %}
     ExceptionState exceptionState(ExceptionState::ExecutionContext, "{{method.name}}", "{{interface_name}}", info.Holder(), info.GetIsolate());
     {% endif %}
     {# Overloaded methods have length checked during overload resolution #}
@@ skipping 103 matching lines @@
     {{cpp_method_call(method, argument.v8_set_return_value_for_main_world, argument.cpp_value) | indent}}
     {% else %}
     {{cpp_method_call(method, argument.v8_set_return_value, argument.cpp_value) | indent}}
     {% endif %}
     {% if argument.has_event_listener_argument %}
     {{hidden_dependency_action(method.name) | indent}}
     {% endif %}
     return;
 }
 {% endif %}
-{% if argument.has_type_checking_interface %}
+{% if argument.has_type_checking_interface and not argument.is_variadic_wrapper_type %}
 {# Type checking for wrapper interface types (if interface not implemented,
-   throw a TypeError), per http://www.w3.org/TR/WebIDL/#es-interface #}
+   throw a TypeError), per http://www.w3.org/TR/WebIDL/#es-interface
+   Note: for variadic arguments, the type checking is done for each matched
+   argument instead; see argument.is_variadic_wrapper_type code-path below. #}
 if (info.Length() > {{argument.index}} && {% if argument.is_nullable %}!isUndefinedOrNull(info[{{argument.index}}]) && {% endif %}!V8{{argument.idl_type}}::hasInstance(info[{{argument.index}}], info.GetIsolate())) {
     {{throw_type_error(method, '"parameter %s is not of type \'%s\'."' %
                                (argument.index + 1, argument.idl_type)) | indent}}
     return;
 }
 {% endif %}{# argument.has_type_checking_interface #}
 {% if argument.is_callback_interface %}
 {# FIXME: remove EventListener special case #}
 {% if argument.idl_type == 'EventListener' %}
 {% if method.name == 'removeEventListener' %}
@@ skipping 399 matching lines @@
 v8::Handle<v8::Object> wrapper = wrap(impl.get(), info.Holder(), isolate);
 {% else %}
 {% set constructor_class = v8_class + ('Constructor'
                                        if constructor.is_named_constructor else
                                        '') %}
 v8::Handle<v8::Object> wrapper = info.Holder();
 V8DOMWrapper::associateObjectWithWrapper<{{v8_class}}>(impl.release(), &{{constructor_class}}::wrapperTypeInfo, wrapper, isolate, {{wrapper_configuration}});
 {% endif %}
 v8SetReturnValue(info, wrapper);
 {% endmacro %}