Make dartj2s do type conversions on constructor arguments

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 dd2cdbdf59e448c82c0093ccc834f16d575705b7..541b5bae75d9e5b45a9c57347e283a0aae938cd7 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -393,6 +393,9 @@ class DartiumBackend(HtmlDartGenerator):
   def CustomJSMembers(self):
     return {}
 
+  def _InputConversion(self, type_name, info):
+    return None
+
   def GenerateCallback(self, info):
     if IsPureInterface(self._interface.id) or IsCustomType(self._interface.id):
       return