Add List.insert.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList implements List {
   // Raw Element.
@@ skipping 196 matching lines @@
 
   int indexOf(Element element, [int start = 0]) {
     return Lists.indexOf(this, element, start, this.length);
   }
 
   int lastIndexOf(Element element, [int start = null]) {
     if (start == null) start = length - 1;
     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 = '';
   }
 
   Element removeAt(int index) {
     final result = this[index];
     if (result != null) {
       _element.$dom_removeChild(result);
     }
@@ skipping 929 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }