Update CustomElement as well.

pkg/custom_element/lib/custom_element.dart

Index: pkg/custom_element/lib/custom_element.dart
diff --git a/pkg/custom_element/lib/custom_element.dart b/pkg/custom_element/lib/custom_element.dart
index db0c3b82d3c87e701e88022f4ae849c560f977b6..202a0eedfc425265d60703dd9be306e6f6f3404a 100644
--- a/pkg/custom_element/lib/custom_element.dart
+++ b/pkg/custom_element/lib/custom_element.dart
@@ -471,17 +471,6 @@ class CustomElement implements Element {
 
   int get scrollWidth => host.scrollWidth;
 
-  String $dom_getAttribute(String name) =>
-      host.$dom_getAttribute(name);
-
-  String $dom_getAttributeNS(String namespaceUri, String localName) =>
-      host.$dom_getAttributeNS(namespaceUri, localName);
-
-  String $dom_setAttributeNS(
-      String namespaceUri, String localName, String value) {
-    host.$dom_setAttributeNS(namespaceUri, localName, value);
-  }
-
   Rect getBoundingClientRect() => host.getBoundingClientRect();
 
   List<Rect> getClientRects() => host.getClientRects();
@@ -489,11 +478,6 @@ class CustomElement implements Element {
   List<Node> getElementsByClassName(String name) =>
       host.getElementsByClassName(name);
 
-  void $dom_setAttribute(String name, String value) =>
-      host.$dom_setAttribute(name, value);
-
-  List<Node> get $dom_childNodes => host.$dom_childNodes;
-
   Node get firstChild => host.firstChild;
 
   Node get lastChild => host.lastChild;