ment specifies when this must happen.</p>
<p>The <code>priority</code>, when provided, must be one of <a href="dart-isolate/Isolate/immediate-constant.html">immediate</a> or
<a href="dart-isolate/Isolate/beforeNextEvent-constant.html">beforeNextEvent</a> (the default).
The shutdown is performed at different times depending on the priority:</p><ul><li><code>immediate</code>: The isolate shuts down as soon as possible.
  Control messages are handled in order, so all previously sent control
  events from this 