Remove deprecated StringBuffer.add, addAll and addCharCode.

samples/swarm/swarm_ui_lib/view/SliderMenu.dart

Index: samples/swarm/swarm_ui_lib/view/SliderMenu.dart
diff --git a/samples/swarm/swarm_ui_lib/view/SliderMenu.dart b/samples/swarm/swarm_ui_lib/view/SliderMenu.dart
index 3eae27ed91bb466f1f9ea2a555ad3c91665cf106..577b84716b3d762395a02b58c3e3e3db1ae7d135 100644
--- a/samples/swarm/swarm_ui_lib/view/SliderMenu.dart
+++ b/samples/swarm/swarm_ui_lib/view/SliderMenu.dart
@@ -36,7 +36,7 @@ class SliderMenu extends View {
     // Create a div for each menu item.
     final items = new StringBuffer();
     for (final item in _menuItems) {
-      items.add('<div class="sm-item">$item</div>');
+      items.write('<div class="sm-item">$item</div>');
     }
 
     // Create a root node to hold this view.