function isPlainObject(value) {
  if (!isObjectLike(value) || baseGetTag(value) != objectTag) {
    return false;
  }
  var proto = getPrototype(value);
  if (proto === null) {
    return true;
  }
  var Ctor = hasOwnProperty.call(proto, 'constructor') && proto.constructor;
  return typeof Ctor == 'function' && Ctor instanceof Ctor &&
    funcToString.call(Ctor) == objectCtorString;
}
Used to compose bitmasks for value comparisons.