Fix attributeChanged signature in WebUI (needs latest cherry-pick of custom

lib/web_ui.dart

Index: lib/web_ui.dart
diff --git a/lib/web_ui.dart b/lib/web_ui.dart
index ab2bf6d696cc810d05fdec37c6ac55a88ae169d3..c79542d104970e17389c1d8503a326f35c937582 100644
--- a/lib/web_ui.dart
+++ b/lib/web_ui.dart
@@ -118,12 +118,6 @@ abstract class WebComponent extends CustomElement {
   /** Invoked when this component is removed from the DOM tree. */
   void removed() {}
 
-  // TODO(jmesserly): how do we implement this efficiently?
-  // See https://github.com/dart-lang/web-ui/issues/37
-  /** Invoked when any attribute of the component is modified. */
-  void attributeChanged(String name, String oldValue) {}
-
-
   /**
    * **Note**: This is an implementation helper and should not need to be called
    * from your code.