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

lib/html/src/DataAttributeMap.dart

Index: lib/html/src/DataAttributeMap.dart
diff --git a/lib/html/src/DataAttributeMap.dart b/lib/html/src/DataAttributeMap.dart
index d20e4fc1a071dd8eb8d999067a9f4eb4f52f55d7..7c754e9fa6e64ff149236bec4a5c98ef51a25958 100644
--- a/lib/html/src/DataAttributeMap.dart
+++ b/lib/html/src/DataAttributeMap.dart
@@ -6,7 +6,7 @@
  * 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;
 
@@ -21,8 +21,8 @@ class _DataAttributeMap implements Map<String, String> {
 
   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()) {