code: private String toJsStringSet(Iterable<String> strings) {
      StringBuilder sb = new StringBuilder();
      boolean isFirst = true;
      sb.append('{');
      for (String str : strings) {
        if (!isFirst) {
          sb.append(", ");
        }
        isFirst = false;
        writeStringLiteral(str, sb);
        sb.append(": true");
      }
      sb.append('}');
      return sb.toString();
    }
docs: ["foo", "bar"] -> '{"foo": true, "bar": true}'