Removing the unused AttributeMap class.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index f7c0641f6433524f4eb4b6192b1549e49e324ad7..b68087fc889e86dfb712ea99d3669d14b3dc11a7 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -9033,16 +9033,7 @@ class _FrozenElementListIterator implements Iterator<Element> {
   bool get hasNext => _index < _list.length;
 }
 
-/**
- * All your attribute manipulation needs in one place.
- * Extends the regular Map interface by automatically coercing non-string
- * values to strings.
- */
-abstract class AttributeMap implements Map<String, String> {
-  void operator []=(String key, value);
-}
-
-class _ElementAttributeMap extends AttributeMap {
+class _ElementAttributeMap implements Map<String, String> {
 
   final Element _element;
 
@@ -9137,7 +9128,7 @@ class _ElementAttributeMap extends AttributeMap {
  * Provides a Map abstraction on top of data-* attributes, similar to the
  * dataSet in the old DOM.
  */
-class _DataAttributeMap extends AttributeMap {
+class _DataAttributeMap implements Map<String, String> {
 
   final Map<String, String> $dom_attributes;
 
@@ -9261,7 +9252,7 @@ class Element extends Node implements ElementTraversal {
    * @domName Element.hasAttribute, Element.getAttribute, Element.setAttribute,
    *   Element.removeAttribute
    */
-  _ElementAttributeMap get attributes => new _ElementAttributeMap(this);
+  Map<String, String> get attributes => new _ElementAttributeMap(this);
 
   void set attributes(Map<String, String> value) {
     Map<String, String> attributes = this.attributes;