Adding tests for Element & Node lists insert method.

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 203 matching lines @@
     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.append(element);
     } else {
-      throw new UnimplementedError("insert on ElementLists");
+      _element.insertBefore(element, this[index]);
     }
   }
 
   void clear() {
     // It is unclear if we want to keep non element nodes?
     _element.text = '';
   }
 
   Element removeAt(int index) {
     final result = this[index];
@@ skipping 906 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');
 }