According to spec it should be a native JavaScript object derived from Error.
Our implementation is an interface.

One way to solve this problem is to implement OverconstrainedError (https://bit.ly/v8-extras) using JavaScript and use V8Extras to be able to access it from Blink.

domenic@ helped us with a spec-compliant (although untested) implementation here: https://gist.github.com/domenic/a4c438614eebd0f04679ee222a884948