Narrowing attribute maps to only accept strings.

tools/dom/src/AttributeMap.dart

 // Copyright (c) 2012, 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.
 
 part of html;
 
 abstract class _AttributeMap implements Map<String, String> {
   final Element _element;
 
   _AttributeMap(this._element);
@@ skipping 72 matching lines @@
   _ElementAttributeMap(Element element): super(element);
 
   bool containsKey(String key) {
     return _element.$dom_hasAttribute(key);
   }
 
   String operator [](String key) {
     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) {
     String value = _element.$dom_getAttribute(key);
     _element.$dom_removeAttribute(key);
     return value;
   }
 
   /**
    * The number of {key, value} pairs in the map.
@@ skipping 15 matching lines @@
   _NamespacedAttributeMap(Element element, this._namespace): super(element);
 
   bool containsKey(String key) {
     return _element.$dom_hasAttributeNS(_namespace, key);
   }
 
   String operator [](String key) {
     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) {
     String value = this[key];
     _element.$dom_removeAttributeNS(_namespace, key);
     return value;
   }
 
   /**
    * The number of {key, value} pairs in the map.
@@ skipping 18 matching lines @@
 
   // interface Map
 
   // TODO: Use lazy iterator when it is available on Map.
   bool containsValue(String value) => values.any((v) => v == value);
 
   bool containsKey(String key) => $dom_attributes.containsKey(_attr(key));
 
   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()) =>
     $dom_attributes.putIfAbsent(_attr(key), ifAbsent);
 
   String remove(String key) => $dom_attributes.remove(_attr(key));
 
   void clear() {
     // Needs to operate on a snapshot since we are mutating the collection.
     for (String key in keys) {
@@ skipping 32 matching lines @@
   int get length => keys.length;
 
   // TODO: Use lazy iterator when it is available on Map.
   bool get isEmpty => length == 0;
 
   // Helpers.
   String _attr(String key) => 'data-$key';
   bool _matches(String key) => key.startsWith('data-');
   String _strip(String key) => key.substring(5);
 }