Narrowing attribute maps to only accept strings.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 7b763024492daee694295d5a872ea84829b6c07d..cf9d96f6bce96f50b7632fa25d5b902cc0442ede 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -30464,8 +30464,8 @@ class _ElementAttributeMap extends _AttributeMap {
     return _element.$dom_getAttribute(key);
   }
 
-  void operator []=(String key, value) {
-    _element.$dom_setAttribute(key, '$value');
+  void operator []=(String key, String value) {
+    _element.$dom_setAttribute(key, value);
   }
 
   String remove(String key) {
@@ -30501,8 +30501,8 @@ class _NamespacedAttributeMap extends _AttributeMap {
     return _element.$dom_getAttributeNS(_namespace, key);
   }
 
-  void operator []=(String key, value) {
-    _element.$dom_setAttributeNS(_namespace, key, '$value');
+  void operator []=(String key, String value) {
+    _element.$dom_setAttributeNS(_namespace, key, value);
   }
 
   String remove(String key) {
@@ -30541,8 +30541,8 @@ class _DataAttributeMap implements Map<String, String> {
 
   String operator [](String key) => $dom_attributes[_attr(key)];
 
-  void operator []=(String key, value) {
-    $dom_attributes[_attr(key)] = '$value';
+  void operator []=(String key, String value) {
+    $dom_attributes[_attr(key)] = value;
   }
 
   String putIfAbsent(String key, String ifAbsent()) =>