The error is not uncatchable. It is directly thrown by Isolate.spawn, which is arguably reasonable for an Error (in this case ArgumentError) - it represents a programming error, which should not happen in a correct program.

It is a problem, though, that there is no way to check whether an object is serializable short of attempting to serialize it, so the Error should perhaps be a non-Error exception instead. In that case, it would make more sense to put it into the future.

More interesting is it that the program hangs after catching the error:

import"dart:isolate";

main() {
 try {
   Isolate.spawn(print, () {}).catchError((e) => print("caught $e"));
 } catch (e) {
   print("catch $e");
 }
 print("Done");
}

This program prints "catch ..." and "Done" and then fails to terminate.

It's very confusing when an asynchronous method synchronously throws errors.

Seems the error now goes into the future returned by Isolate.spawn.

Added Fixed label.