Add Observatory test of io.

tests/standalone/io/socket_exception_test.dart

 // Copyright (c) 2013, 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 socket exceptions.
 
 import "dart:async";
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
@@ skipping 189 matching lines @@
     ServerSocket.bind("127.0.0.1", 0).then((server) {
       var completer = new Completer();
       server.listen((socket) {
         completer.future.then((_) => socket.destroy());
       });
       Socket.connect("127.0.0.1", server.port).then((client) {
         const int SIZE = 1024 * 1024;
         int errors = 0;
         client.add(new List.filled(SIZE, 0));
         client.close();
-        client.done.catchError((error) {
-          server.close();
-        });
+        client.done
+            .catchError((_) {})
+            .whenComplete(() {
+              server.close();
+            });
         // Destroy other socket now.
         completer.complete(null);
       });
     });
   }
 
   static void unknownHostTest() {
     asyncStart();
     Socket.connect("hede.hule.hest", 1234)
         .then((socket) => Expect.fail("Connection completed"))
@@ skipping 14 matching lines @@
     unknownHostTest();
   }
 }
 
 main() {
   asyncStart();
   SocketExceptionTest.testMain();
   asyncEnd();
 }