Remove deprecated StringBuffer.add, addAll and addCharCode.

runtime/vm/snapshot_test.dart

Index: runtime/vm/snapshot_test.dart
diff --git a/runtime/vm/snapshot_test.dart b/runtime/vm/snapshot_test.dart
index 9742c38bc90f12b031d21898158b804d65ac9ab2..50ed111e48eb1c8ea922ed146d739fdd23cb154e 100644
--- a/runtime/vm/snapshot_test.dart
+++ b/runtime/vm/snapshot_test.dart
@@ -1483,10 +1483,10 @@ 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);
   }