Rework overloads/optionals dispatch to use new ?a syntax.

lib/html/scripts/systemhtml.py

Index: lib/html/scripts/systemhtml.py
diff --git a/lib/html/scripts/systemhtml.py b/lib/html/scripts/systemhtml.py
index 2abfed2d85d1edecf4a0c69e84a2ab30b616046f..93c852e51cb8d2621b4fc2a37a86dac5c2454d2f 100644
--- a/lib/html/scripts/systemhtml.py
+++ b/lib/html/scripts/systemhtml.py
@@ -1076,7 +1076,7 @@ class HtmlDart2JSClassGenerator(Dart2JSInterfaceGenerator):
         '  }\n',
         TYPE=return_type,
         HTML_NAME=html_name,
-        PARAMS=info.ParametersImplementationDeclaration(InputType, '_default'))
+        PARAMS=info.ParametersImplementationDeclaration(InputType))
 
     parameter_names = [param_info.name for param_info in info.param_infos]
     parameter_types = [InputType(param_info.dart_type)
@@ -1153,19 +1153,19 @@ class HtmlDart2JSClassGenerator(Dart2JSInterfaceGenerator):
           NATIVE=info.declared_name)
 
     def GenerateChecksAndCall(operation, argument_count):
-      checks = ['_default == %s' % name for name in parameter_names]
+      checks = ['!?%s' % name for name in parameter_names]
       for i in range(0, argument_count):
         argument = operation.arguments[i]
         parameter_name = parameter_names[i]
         test_type = self._DartType(argument.type.id)
         if test_type in ['Dynamic', 'Object']:
-          checks[i] = '_default != %s' % parameter_name
+          checks[i] = '?%s' % parameter_name
         elif test_type == parameter_types[i]:
           checks[i] = 'true'
         else:
-          checks[i] = '(%s is %s || %s == null)' % (
+          checks[i] = '(%s is %s || %s === null)' % (
               parameter_name, test_type, parameter_name)
-      # There can be multiple _default checks.  We need them all since a later
+      # There can be multiple presence checks.  We need them all since a later
       # optional argument could have been passed by name, leaving 'holes'.
       GenerateCall(operation, argument_count, checks)
 
@@ -1184,7 +1184,7 @@ class HtmlDart2JSClassGenerator(Dart2JSInterfaceGenerator):
       argument_count = len(operation.arguments)
       for position, argument in list(enumerate(operation.arguments))[::-1]:
         if self._IsOptional(operation, argument):
-          check = '_default != %s' % parameter_names[position]
+          check = '?%s' % parameter_names[position]
           GenerateCall(operation, position + 1, [check])
           argument_count = position
       GenerateCall(operation, argument_count, [])