Allow null strings in setters.

tools/dom/scripts/systemnative.py

Index: tools/dom/scripts/systemnative.py
diff --git a/tools/dom/scripts/systemnative.py b/tools/dom/scripts/systemnative.py
index 8b8e82576fdeca27193a38a8cf7e221beb49921a..c016b7ebe92c19d9d4cd2ead190b974ce2425473 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -10,7 +10,7 @@ import emitter
 import os
 from generator import *
 from htmldartgenerator import *
-from idlnode import IDLArgument
+from idlnode import IDLArgument, IDLAttribute
 from systemhtml import js_support_checks, GetCallbackInfo, HTML_LIBRARY_NAMES
 
 _cpp_type_map = {
@@ -798,6 +798,10 @@ class DartiumBackend(HtmlDartGenerator):
         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