Chrome roll.

tools/dom/scripts/systemhtml.py

Index: tools/dom/scripts/systemhtml.py
diff --git a/tools/dom/scripts/systemhtml.py b/tools/dom/scripts/systemhtml.py
index f88a451c61bf642b44455a0b731248ca34cec91b..d6fa18ad17b00129ba331222afa211b0f6dbcefb 100644
--- a/tools/dom/scripts/systemhtml.py
+++ b/tools/dom/scripts/systemhtml.py
@@ -651,7 +651,7 @@ class Dart2JSBackend(HtmlDartGenerator):
     return self._interface.doc_js_name in _js_custom_constructors
 
   def IsConstructorArgumentOptional(self, argument):
-    return 'Optional' in argument.ext_attrs
+    return argument.optional
 
   def EmitStaticFactoryOverload(self, constructor_info, name, arguments):
     index = len(arguments)
@@ -994,7 +994,7 @@ class Dart2JSBackend(HtmlDartGenerator):
         parameter_names,
         declaration,
         GenerateCall,
-        self._IsOptional,
+        lambda _, argument: IsOptional(argument),
         can_omit_type_check=lambda type, pos: type == parameter_types[pos])
 
   def _AddInterfaceOperation(self, info, html_name):
@@ -1005,9 +1005,6 @@ class Dart2JSBackend(HtmlDartGenerator):
         NAME=info.name,
         PARAMS=info.ParametersDeclaration(self._NarrowInputType))
 
-  def _IsOptional(self, operation, argument):
-    return IsOptional(argument)
-
 
   def _OperationRequiresConversions(self, operation):
     return (self._OperationRequiresOutputConversion(operation) or