Fix bug 15417

tools/dom/templates/html/impl/impl_Storage.darttemplate

Index: tools/dom/templates/html/impl/impl_Storage.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Storage.darttemplate b/tools/dom/templates/html/impl/impl_Storage.darttemplate
index d9b0e1ae74f9052bbdd760d4c094f11bc14884f0..82d1f0c50a113fce01cc288a35d180b413baf801 100644
--- a/tools/dom/templates/html/impl/impl_Storage.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Storage.darttemplate
@@ -32,6 +32,10 @@ part of $LIBRARYNAME;
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS
     implements Map<String, String> $NATIVESPEC {
 
+  void addAll(Map<String, String> other) {
+    other.forEach((k, v) { this[k] = v; });
+  }
+
   // TODO(nweiz): update this when maps support lazy iteration
   bool containsValue(String value) => values.any((e) => e == value);