Add List.insert.

sdk/lib/html/html_common/filtered_element_list.dart

Index: sdk/lib/html/html_common/filtered_element_list.dart
diff --git a/sdk/lib/html/html_common/filtered_element_list.dart b/sdk/lib/html/html_common/filtered_element_list.dart
index 2883cf76d9535885057eb8b0cee03959315b171b..2429d0e4662d3c267be98b9f0cfc64653e6c894e 100644
--- a/sdk/lib/html/html_common/filtered_element_list.dart
+++ b/sdk/lib/html/html_common/filtered_element_list.dart
@@ -105,6 +105,10 @@ class FilteredElementList implements List {
   Iterable<Element> where(bool f(Element element)) => _filtered.where(f);
   Iterable expand(Iterable f(Element element)) => _filtered.expand(f);
 
+  void insertAt(int index, Element value) {
+    _childNodes.insertAt(index, value);
+  }
+
   Element removeAt(int index) {
     final result = this[index];
     result.remove();