Tweak isolate/spawn_uri_missing*test.dart

dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Tests that Isolate.spawnUri completes with an error when the given URI
 /// doesn't resolve to an existing resource.
 ///
 /// This test is similar to spawn_uri_missing_test.dart, but tests what happens
 /// when Isolate.spawnUri is called from an a spawned isolate.  In dart2js,
 /// these two situations are different.
@@ skipping 12 matching lines @@
 void isolate(SendPort port) {
   doTest().then(
       (_) => port.send(SUCCESS),
       onError: (error, stack) => port.send('Test failed: $error\n$stack'));
 }
 
 main() {
   ReceivePort port = new ReceivePort();
   Isolate.spawn(isolate, port.sendPort);
   Completer completer = new Completer();
-  port.listen((message) {
+  port.first.then((message) {
     if (message == SUCCESS) {
       completer.complete(null);
     } else {
       completer.completeError(message);
     }
-    port.close();
   });
 
   asyncTest(() => completer.future);
 }