Fix issue http://code.google.com/p/dart/issues/detail?id=1877 without degrading performance. Impro…

lib/html/src/DataAttributeMap.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Provides a Map abstraction on top of data-* attributes, similar to the
  * dataSet in the old DOM.
  */
-class _DataAttributeMap implements Map<String, String> {
+class _DataAttributeMap implements AttributeMap {
 
   final Map<String, String> _attributes;
 
   _DataAttributeMap(this._attributes);
 
   // interface Map
 
   // TODO: Use lazy iterator when it is available on Map.
   bool containsValue(String value) => getValues().some((v) => v == value);
 
   bool containsKey(String key) => _attributes.containsKey(_attr(key));
 
   String operator [](String key) => _attributes[_attr(key)];
 
-  void operator []=(String key, String value) {
-    _attributes[_attr(key)] = value;
+  void operator []=(String key, value) {
+    _attributes[_attr(key)] = '$value';
   }
 
   String putIfAbsent(String key, String ifAbsent()) {
     if (!containsKey(key)) {
       return this[key] = ifAbsent();
     }
     return this[key];
   }
 
   String remove(String key) => _attributes.remove(_attr(key));
@@ skipping 37 matching lines @@
 
   // TODO: Use lazy iterator when it is available on Map.
   bool isEmpty() => length == 0;
 
   // Helpers.
   String _attr(String key) => 'data-$key';
   bool _matches(String key) => key.startsWith('data-');
   String _strip(String key) => key.substring(5);
 }