Fix analyzer warnings in service tests

runtime/observatory/tests/service/get_retaining_path_rpc_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 'dart:async';
-
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
 class _TestClass {
   _TestClass(this.x, this.y);
   var x;
   var y;
 }
@@ skipping 78 matching lines @@
       'limit': 4,
     };
     var result = await isolate.invokeRpcNoUpgrade('_getRetainingPath', params);
     expect(result['type'], equals('RetainingPath'));
     expect(result['elements'][0]['parentListIndex'], equals(12));
     expect(result['elements'][2]['value']['name'], equals('globalList'));
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:warmup);