Dart: Better handle leak checks. close() is async.

services/dart/dart_apptests/echo_apptests.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+library echo_apptests;
+
 import 'dart:async';
 import 'mojo:application';
 import 'mojo:bindings';
 import 'mojo:core';
 
 import 'package:apptest/apptest.dart';
 import 'package:services/dart/test/echo_service.mojom.dart';
 
 echoApptests(Application application, String url) {
   group('Echo Service Apptests', () {
@@ skipping 30 matching lines @@
       var v = await echoProxy.ptr.echoString(null);
       expect(v.value, equals(null));
 
       var q = await echoProxy.ptr.echoString("quit");
       expect(q.value, equals("quit"));
 
       echoProxy.close();
     });
   });
 }