Add List.insert.

tools/dom/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
   // From Iterable<$E>:
 
   Iterator<$E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
     // be cached in both iterator _and_ forEach method. For now caching it
     // for consistency.
     return new FixedSizeListIterator<$E>(this);
@@ skipping 126 matching lines @@
     if (length == 0) throw new StateError("No elements");
     throw new StateError("More than one element");
   }
 
   $E min([int compare($E a, $E b)]) =>
       IterableMixinWorkaround.min(this, compare);
 
   $E max([int compare($E a, $E b)]) =>
       IterableMixinWorkaround.max(this, compare);
 
+  void insert(int index, $E element) {
+    throw new UnsupportedError("Cannot add to immutable List.");
+  }
+
   $E removeAt(int pos) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   $E removeLast() {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   void remove(Object object) {
     throw new UnsupportedError("Cannot remove from immutable List.");
@@ skipping 27 matching lines @@
     throw new UnsupportedError("Cannot insertRange on immutable List.");
   }
 
   List<$E> getRange(int start, int rangeLength) =>
       Lists.getRange(this, start, rangeLength, <$E>[]);
 
   Map<int, $E> asMap() =>
     IterableMixinWorkaround.asMapList(this);
 
   // -- end List<$E> mixins.