Add List.insert.

tools/dom/src/WrappedList.dart

Index: tools/dom/src/WrappedList.dart
diff --git a/tools/dom/src/WrappedList.dart b/tools/dom/src/WrappedList.dart
index ecad3061f6057a9e9620c7f65d8ab8b80bc4c38e..4062c7a537acdf0f0a330a91fd150c7664e6f3b4 100644
--- a/tools/dom/src/WrappedList.dart
+++ b/tools/dom/src/WrappedList.dart
@@ -109,6 +109,8 @@ class _WrappedList<E> implements List<E> {
 
   int lastIndexOf(E element, [int start]) => _list.lastIndexOf(element, start);
 
+  void insertAt(int index, E element) => _list.insertAt(index, element);
+
   E removeAt(int index) => _list.removeAt(index);
 
   E removeLast() => _list.removeLast();