function validateLabel(label, useStd3ASCII, transitional) {
  // 2. The label must not contain a U+002D HYPHEN-MINUS character in both the
  // third position and fourth positions.
  if (label[2] == '-' && label[3] == '-')
    throw new Error("Failed to validate " + label);

  // 3. The label must neither begin nor end with a U+002D HYPHEN-MINUS
  // character.
  if (label.startsWith('-') || label.endsWith('-'))
    throw new Error("Failed to validate " + label);

  // 4. The label must not contain a U+002E ( . ) FULL STOP.
  if (label.includes('.'))
    throw new Error("Failed to validate " + label);

  if (mapLabel(label, useStd3ASCII, transitional) != label)
    throw new Error("Failed to validate " + label);

  // 5. The label must not begin with a combining mark, that is:
  // General_Category=Mark.
  var ch = label.codePointAt(0);
  if (idna_map.mapChar(ch) & (0x2 << 23))
    throw new Error("Label contains illegal character: " + ch);
}
A singleton instance for a MessagePort to the extension.
@type {MessagePort|u2f.WrappedChromeRuntimePort_}
@private