The HTML spec says that all exotic objects should be rejected:
  https://html.spec.whatwg.org/#safe-passing-of-structured-data

The current ECMAScript draft says that objects immutable prototype exotic objects, like Object.prototype, meet that criterion.

Therefore it seems we should reject such objects (or change the HTML specification). I am confident that attempts to serialize such prototypes are rare.

All it should take is checking map()->is_immutable_proto() and bailing if it's true.