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

lib/html/templates/html/dart2js/html_dart2js.darttemplate

Index: lib/html/templates/html/dart2js/html_dart2js.darttemplate
diff --git a/lib/html/templates/html/dart2js/html_dart2js.darttemplate b/lib/html/templates/html/dart2js/html_dart2js.darttemplate
index d331b1afe427ab96bc3d2fd5d0f0ca7d5f585842..b05b5756f0d1a1484168a5bdec2beb307032fdfc 100644
--- a/lib/html/templates/html/dart2js/html_dart2js.darttemplate
+++ b/lib/html/templates/html/dart2js/html_dart2js.darttemplate
@@ -45,13 +45,6 @@ _DocumentImpl get _document() native "return document;";
 Element query(String selector) => _document.query(selector);
 ElementList queryAll(String selector) => _document.queryAll(selector);
 
-/// Marker for defaulted arguments.
-class _Default {
-  const _Default();
-}
-
-final _default = const _Default();
-
 // Workaround for tags like <cite> that lack their own Element subclass --
 // Dart issue 1990.
 class _HTMLElementImpl extends _ElementImpl native "*HTMLElement" {