title: Flutter Isolate receives only the initial message
body: <p>I'm trying to use an <code>Isolate</code> that will send a request, so as not to block the main thread/isolate.</p>
  <p>After using so, the function of the spawned <code>Isolate</code> is called only once, on the initial passed message.</p>
  <pre class="lang-dart prettyprint-override"><code>final isolate = await Isolate.spawn&lt;Animal&gt;((animal) {
    print('received ${animal.name}');
  }, Animal(&quot;Foo&quot;));
  </code></pre>
  <p>Prints: <code>flutter: received Foo</code></p>
  <p>But adding a few more shows nothing:</p>
  <pre class="lang-dart prettyprint-override"><code>print('Sending bar');
  isolate.controlPort.send(Animal(&quot;Bar&quot;));
  
  print('Sending test');
  isolate.controlPort.send(Animal(&quot;Test&quot;));
  
  sleep(const Duration(seconds: 10));
  </code></pre>
  <pre><code>flutter: Sending bar
  flutter: Sending test
  flutter: received Foo
  </code></pre>