Omit '= null' as default value in generated dart:html code.

lib/dom/scripts/generator.py

Index: lib/dom/scripts/generator.py
diff --git a/lib/dom/scripts/generator.py b/lib/dom/scripts/generator.py
index dd5b2eb15f09cb5cbdf8c5567558cbb09f5f1c7c..2a8bab7544a98327368403e5c4cf80aac889055a 100644
--- a/lib/dom/scripts/generator.py
+++ b/lib/dom/scripts/generator.py
@@ -425,7 +425,7 @@ class OperationInfo(object):
     def FormatParam(param):
       """Returns a parameter declaration fragment for an ParamInfo."""
       type = type_fn(param)
-      if param.is_optional and default_value:
+      if param.is_optional and default_value and default_value != 'null':
         return '%s%s = %s' % (type, param.name, default_value)
       return '%s%s' % (type, param.name)