Add List.insert.

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 3556d5bbcbd5e8612decf3547069fd8032015208..3e7f6a4c1844b566b4eb8e25c321f5b12a651adf 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -214,6 +214,17 @@ class _ChildrenElementList implements List {
     return Lists.lastIndexOf(this, element, start);
   }
 
+  void insert(int index, Element element) {
+    if (index < 0 || index > length) {
+      throw new RangeError.range(index, 0, length);
+    }
+    if (index == length) {
+      _element.$dom_appendChild(element);
+    } else {
+      throw new UnimplementedError("insert on ElementLists");
+    }
+  }
+
   void clear() {
     // It is unclear if we want to keep non element nodes?
     _element.text = '';