Make VM event streams look like real dart streams.

runtime/observatory/tests/service/library_dependency_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--compile-all --error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
-import 'package:observatory/debugger.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
-import 'dart:async';
 
 export 'dart:collection';
 import 'dart:mirrors' as mirrors;
 import 'dart:convert' deferred as convert;
 
 var tests = [
 
 (Isolate isolate) async {
   var lib = await isolate.rootLibrary.load();
 
@@ skipping 12 matching lines @@
   expect(importOf("dart:mirrors").prefix, equals("mirrors"));
 
   expect(importOf("dart:convert").isImport, isTrue);
   expect(importOf("dart:convert").isExport, isFalse);
   expect(importOf("dart:convert").isDeferred, isTrue);
   expect(importOf("dart:convert").prefix, equals("convert"));
 },
 ];
 
 main(args) => runIsolateTests(args, tests);