e that uses callbacks, the
callback must be registered using <a href="dart-async/Zone/registerCallback.html">registerCallback</a> at the point where the
user provides the callback. This allows zones to record other information
that they need at the same time, perhaps even wrapping the callback, so
that the callback is prepared when it is later run in the same zones
(using <a href="dart-async/Zone/run.html">run</a>). For example, a zone may decide
to store the stack trace (at the time of the registration) wi