code> can be: primitive values (null, num, bool, double,
String), instances of <a href="dart-isolate/SendPort-class.html">SendPort</a>, and lists and maps whose elements are any
of these. List and maps are also allowed to be cyclic.</p>
<p>In the special circumstances when two isolates share the same code and are
running in the same process (e.g. isolates created via <a href="dart-isolate/Isolate/spawn.html">Isolate.spawn</a>), it
is also possible to send object instances (which would be copied in the
proce