Add List.insert.

tools/dom/templates/immutable_list_mixin.darttemplate

Index: tools/dom/templates/immutable_list_mixin.darttemplate
diff --git a/tools/dom/templates/immutable_list_mixin.darttemplate b/tools/dom/templates/immutable_list_mixin.darttemplate
index 7f2497fd03c8e0bcb8f08473380504427662eb81..697f24368a7077a66860ffc32c7afe553259bb2a 100644
--- a/tools/dom/templates/immutable_list_mixin.darttemplate
+++ b/tools/dom/templates/immutable_list_mixin.darttemplate
@@ -144,6 +144,10 @@ $endif
   $E max([int compare($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.");
   }