Added expand method to iterables.

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

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 58c63f52d6e535a4761bafab31b82fd3f175242f..ed07849f7d68c57951b5753827d02e41cb4b4c9f 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -73,6 +73,10 @@ class _ChildrenElementList implements List {
     return IterableMixinWorkaround.where(this, f);
   }
 
+  Iterable expand(Iterable f(Element element)) {
+    return IterableMixinWorkaround.expand(this, f);
+  }
+
   bool get isEmpty {
     return _element.$dom_firstElementChild == null;
   }
@@ -290,6 +294,10 @@ class _FrozenElementList implements List {
     return IterableMixinWorkaround.where(this, f);
   }
 
+  Iterable expand(Iterable f(Element element)) {
+    return IterableMixinWorkaround.expand(this, f);
+  }
+
   bool every(bool f(Element element)) {
     for(Element element in this) {
       if (!f(element)) {