Remove deprecated StringBuffer.add, addAll and addCharCode.

samples/swarm/swarm_ui_lib/view/MeasureText.dart

Index: samples/swarm/swarm_ui_lib/view/MeasureText.dart
diff --git a/samples/swarm/swarm_ui_lib/view/MeasureText.dart b/samples/swarm/swarm_ui_lib/view/MeasureText.dart
index 3cc1970ab801eb58332b04a05c5d3eca937436f2..e0efed8a55c390897bbf4ed0e331634a02f6e434 100644
--- a/samples/swarm/swarm_ui_lib/view/MeasureText.dart
+++ b/samples/swarm/swarm_ui_lib/view/MeasureText.dart
@@ -103,10 +103,10 @@ class MeasureText {
       }
       if (sb != null) {
         if (lines > 1) {
-          sb.add('<br>');
+          sb.write('<br>');
         }
         // TODO(jacobr): HTML escape this text.
-        sb.add(text.substring(start, end));
+        sb.write(text.substring(start, end));
       }
     });
     return lines;