adoptStdStrings(stdStrings) {
    this.write(stdStrings.stdout);
    // stderr accrues many empty lines. Don't log unless there's content.
    if (/\S/.test(stdStrings.stderr)) {
      this.write(stdStrings.stderr);
    }
  }
Append a stdout and stderr to this log.
@param {{stdout: string, stderr: string}} stdStrings