Tweak isolate/spawn_uri_missing*test.dart

dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart

Index: dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart
diff --git a/dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart b/dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart
index daefc46e3818cd5906b774565f465b1a7b0e8df9..c7551d2b08f55fdd60e84b3c5e08f396760daabd 100644
--- a/dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart
+++ b/dart/tests/isolate/spawn_uri_missing_from_isolate_test.dart
@@ -30,13 +30,12 @@ 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);