Fix analyzer warnings in service tests

runtime/observatory/tests/service/get_isolate_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';
 
 var tests = [
   (VM vm) async {
     var params = {
       'isolateId': vm.isolates.first.id,
     };
@@ skipping 11 matching lines @@
     expect(result['libraries'][0]['type'], equals('@Library'));
     expect(result['breakpoints'].length, isZero);
     expect(result['features'].length, isPositive);
     expect(result['features'][0], new isInstanceOf<String>());
     expect(result['heaps']['new']['type'], equals('HeapSpace'));
     expect(result['heaps']['old']['type'], equals('HeapSpace'));
   },
 ];
 
 main(args) async => runVMTests(args, tests);