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

lib/html/templates/html/dartium/html_dartium.darttemplate

Index: lib/html/templates/html/dartium/html_dartium.darttemplate
diff --git a/lib/html/templates/html/dartium/html_dartium.darttemplate b/lib/html/templates/html/dartium/html_dartium.darttemplate
index 842b36a88ae95d5895857f8d8d5e3a85647aa6f3..3c5cbc3df873cce0b7c06d7fa873236708d2f887 100644
--- a/lib/html/templates/html/dartium/html_dartium.darttemplate
+++ b/lib/html/templates/html/dartium/html_dartium.darttemplate
@@ -60,12 +60,6 @@ Document get _document() => _window.document;
 Element query(String selector) => _document.query(selector);
 ElementList queryAll(String selector) => _document.queryAll(selector);
 
-class _Null {
-  const _Null();
-}
-
-final _null = const _Null();
-
 int _getNewIsolateId() => _Utils._getNewIsolateId();
 
 bool _callPortInitialized = false;