Fix analyzer warnings in service tests

runtime/observatory/tests/service/get_object_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
 
 library get_object_rpc_test;
 
-import 'dart:async';
-
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_helper.dart';
 
 class _DummyClass {
   static var dummyVar = 11;
   void dummyFunction() {
   }
 }
 
 class _DummySubClass extends _DummyClass {
 }
 
 void warmup() {
+  // Silence analyzer.
+  new _DummySubClass();
   new _DummyClass().dummyFunction();
 }
 
 eval(Isolate isolate, String expression) async {
   Map params = {
     'targetId': isolate.rootLib.id,
     'expression': expression,
   };
   return await isolate.invokeRpcNoUpgrade('eval', params);
 }
@@ skipping 325 matching lines @@
     var params = {
       'objectId': 'code/0',
     };
     var result = await isolate.invokeRpcNoUpgrade('getObject', params);
     expect(result['type'], equals('Error'));
     expect(result['message'], startsWith('Unrecognized object id'));
   },
 ];
 
 main(args) async => runIsolateTests(args, tests, testeeBefore:warmup);