Fix bug 15417

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 b74110b2e3393bd192243d350653d458cf1e5dc8..c212a23a840fc1c03e12c16e5a1764ad0c894955 100644
--- a/tools/dom/src/AttributeMap.dart
+++ b/tools/dom/src/AttributeMap.dart
@@ -9,6 +9,10 @@ abstract class _AttributeMap implements Map<String, String> {
 
   _AttributeMap(this._element);
 
+  void addAll(Map<String, String> other) {
+    other.forEach((k, v) { this[k] = v; });
+  }
+
   bool containsValue(String value) {
     for (var v in this.values) {
       if (value == v) {
@@ -165,6 +169,10 @@ class _DataAttributeMap implements Map<String, String> {
 
   // interface Map
 
+  void addAll(Map<String, String> other) {
+    other.forEach((k, v) { this[k] = v; });
+  }
+
   // TODO: Use lazy iterator when it is available on Map.
   bool containsValue(String value) => values.any((v) => v == value);