Add List.insert.

samples/swarm/swarm_ui_lib/observable/observable.dart

Index: samples/swarm/swarm_ui_lib/observable/observable.dart
diff --git a/samples/swarm/swarm_ui_lib/observable/observable.dart b/samples/swarm/swarm_ui_lib/observable/observable.dart
index e60b8a92ee23585f7eca59833ebcd1a5cb3f3c35..db02b3e8ee86cadf6f17c2f355e35eb234d42fe4 100644
--- a/samples/swarm/swarm_ui_lib/observable/observable.dart
+++ b/samples/swarm/swarm_ui_lib/observable/observable.dart
@@ -198,6 +198,11 @@ class ObservableList<T>
   T min([int compare(T a, T b)]) => _internal.min(compare);
   T max([int compare(T a, T b)]) => _internal.max(compare);
 
+  void insert(int index, T element) {
+    _internal.insert(index, element);
+    recordListInsert(index, element);
+  }
+
   T removeLast() {
     final result = _internal.removeLast();
     recordListRemove(length, result);