Port of github.com/polymer/polymer.

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 d3bf02f1d27104b8e5c4de0a35aad79934aa4d8f..da705895fb5d7639be137543f705db34da7b3327 100644
--- a/pkg/custom_element/lib/custom_element.dart
+++ b/pkg/custom_element/lib/custom_element.dart
@@ -23,6 +23,8 @@ import 'package:mdv/mdv.dart' as mdv;
 import 'package:meta/meta.dart';
 import 'src/custom_tag_name.dart';
 
+part 'src/attribute_map.dart';
+
 // TODO(jmesserly): replace with a real custom element polyfill.
 // This is just something temporary.
 /**
@@ -115,6 +117,7 @@ class CustomElement implements Element {
   /** The web component element wrapped by this class. */
   Element _host;
   List _shadowRoots;
+  _AttributeMap _attributes;
 
   /**
    * Shadow roots generated by dwc for each custom element, indexed by the
@@ -178,10 +181,8 @@ class CustomElement implements Element {
   /** 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, String newValue) {}
+  void attributeChanged(String name, String oldValue) {}
 
   get model => host.model;
 
@@ -268,9 +269,12 @@ class CustomElement implements Element {
   Node insertAllBefore(Iterable<Node> newChild, Node refChild) =>
     host.insertAllBefore(newChild, refChild);
 
-  Map<String, String> get attributes => host.attributes;
+  Map<String, String> get attributes {
+    if (_attributes == null) _attributes = new _AttributeMap(this);
+    return _attributes;
+  }
   set attributes(Map<String, String> value) {
-    host.attributes = value;
+    (attributes as _AttributeMap)._replaceAll(value);
   }
 
   List<Element> get elements => host.children;