Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/isolate/mandel_isolate_test.dart

Index: tests/isolate/mandel_isolate_test.dart
diff --git a/tests/isolate/mandel_isolate_test.dart b/tests/isolate/mandel_isolate_test.dart
index abc4d8315215dc90259d593ce9faba1b3750dd2d..3e0354475fe5385c5cf0454cb5be1a830343a0f4 100644
--- a/tests/isolate/mandel_isolate_test.dart
+++ b/tests/isolate/mandel_isolate_test.dart
@@ -67,10 +67,10 @@ class MandelbrotState {
     for (int i = 0; i < _result.length; i++) {
       List<int> line = _result[i];
       for (int j = 0; j < line.length; j++) {
-        if (line[j] < 10) output.add("0");
-        output.add(line[j]);
+        if (line[j] < 10) output.write("0");
+        output.write(line[j]);
       }
-      output.add("\n");
+      output.write("\n");
     }
     // print(output);
   }