Narrowing attribute maps to only accept strings.

tools/dom/src/AttributeMap.dart

Index: tools/dom/src/AttributeMap.dart
diff --git a/tools/dom/src/AttributeMap.dart b/tools/dom/src/AttributeMap.dart
index d309a71818aa499b937dea908e4c7224062e0d11..1d196e36d2d17e952176fa2cc2abadb6af0ef939 100644
--- a/tools/dom/src/AttributeMap.dart
+++ b/tools/dom/src/AttributeMap.dart
@@ -90,8 +90,8 @@ 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) {
@@ -127,8 +127,8 @@ 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) {
@@ -167,8 +167,8 @@ 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()) =>