Adding tests for Element & Node lists insert method.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index eb03c9f02459e6392e8d86b56782968827cd6900..e499dbaffde3b29b54f0af63c8edcdf1b9afc1eb 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -8208,7 +8208,7 @@ class _ChildrenElementList implements List {
     if (index == length) {
       _element.append(element);
     } else {
-      throw new UnimplementedError("insert on ElementLists");
+      _element.insertBefore(element, this[index]);
     }
   }
 
@@ -17433,7 +17433,7 @@ class _ChildNodeListLazy implements List {
     if (index == length) {
       _this.append(node);
     } else {
-      this_.insertBefore(node, this[index]);
+      _this.insertBefore(node, this[index]);
     }
   }