docs: public String decodeJsonLiteral() {
          // This decodes the string from UTF_8 bytes.
          StringBuilder out = new StringBuilder(len);
  
          boolean esc = false;
          final int end = off + len - 1;
          for (int i = off + 1; i < end; ++i) {
              char ch = fb[i];
              if (esc) {
                  esc = false;
  
                  switch (ch) {
                      case 'b':
                          out.append('\b');
                          break;
                      case 'f':
                          out.append('\f');
                          break;
                      case 'n':
                          out.append('\n');
                          break;
                      case 'r':
                          out.append('\r');
                          break;
                      case 't':
                          out.append('\t');
                          break;
                      case '\"':
                      case '\\':
                      case '/':
                          out.append(ch);
                          break;
                      case 'u':
                          int endU = i + 5;
                          if (end < endU) {
                              out.append('?');
                          } else {
                              int n = 0;
                              int pos = i + 1;
                              for (; pos < endU; ++pos) {
                                  ch = fb[pos];
                                  if (ch >= '0' && ch <= '9') {
                                      n = (n << 4) + (ch - '0');
                                  } else if (ch >= 'a' && ch <= 'f') {
                                      n = (n << 4) + (ch - ('a' - 10));
                                  } else if (ch >= 'A' && ch <= 'F') {
                                      n = (n << 4) + (ch - ('A' - 10));
                                  } else {
                                      n = '?';
                                      break;
                                  }
                              }
                              out.append((char) n);
                          }
                          i += 4;  // skipping 4 more characters.
                          break;
                      default:
                          out.append('?');
                          break;
                  }
              } else if (ch == '\\') {
                  esc = true;
              } else {
                  out.append(ch);
              }
          }
          return out.toString();
      }