I don't know if you did wrong, it could be a bug. But it seems exceptions thrown in the isolate's main function aren't caught by the handler. If you change it like this:
import 'dart:isolate';
void isolateMain() {
port.receive((whatever, mahPort) {
throw new Exception("$whatever");
});
}
bool handleException(IsolateUnhandledException e) {
print("EXCEPTION in isolate: ${e.toString()}");
return true;
}
void main() {
SendPort sendPort = spawnFunction(isolateMain, handleException);
sendPort.call("Hello").then((e) {
print("Main received $e");
});
}
... then handleException()
will be called.