 isolate entry functions (<code>main</code> or spawned functions) start running in
the root zone (that is, <a href="dart-async/Zone/current.html">Zone.current</a> is identical to <a href="dart-async/Zone/root-constant.html">Zone.root</a> when the
entry function is called). If no custom zone is created, the rest of the
program always runs in the root zone.</p>
<p>The root zone implements the default behavior of all zone operations.
Many methods, like <a href="dart-async/Zone/registerCallback.html">registerCa